Polysaccharide Dispersions: Chemistry and Technology in Food (Food Science and Technology)

POLYSACCHARIDE DISPERSIONS: CHEMISTRY AND TECHNOLOGY IN FOOD

FOOD SCIENCE AND TECHNOLOGY International Series SERIES EDITOR Steve L. Taylor

University of Nebraska ADVISORY BOARD Daryl B. Lund

Cornell University Douglas Archer

Susan K. Harlander

FDA, Washington, D C

Land O'Lakes, Inc.

Jesse F. Gregory, III

Barbara O. Schneeman

University of Florida

University of California, Davis

A complete list of the books in this series appears at the end of the volume.

Polysaccharide Dispersions: Chemistry and Technology in Food Reginald H. Walter Departmentof Food Science and Technology Cornell University Geneva, New York

ACADEMIC PRESS San Diego

London

Boston

New York Sydney Tokyo ii

Toronto

This book is printed on acid-free paper.

Copyright 9 1998 by ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Academic Press a division of Harcourt Brace & Company 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.apnet.com Academic Press Limited 24-28 Oval Road, London NW1 7DX, UK http://www.hbuk.co.uk/ap/ Library of Congress Card Catalog Number: 97-074420 International Stanrdard Book Number: 0-12-733865-9

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Contents

xi

Preface

Symbols and Abbreviations

xiii

CHAPTER I

Origin and Characteristics of Polysaccharides I. Introduction I II. Physical-Chemical State A. B. C. D. E.

3

Molecular Weight and Degree of Polymerization Configurations and Conformations 6 Colloidal Activity 12 Heterogeneity and Homogeneity 18 Polymorphism, Hysteresis, and Syneresis 18

III. Phenomenology 19 IV. Property and Function Modifications A. B. C. D. E. F. G. H.

Acid 2I Alkali 2I Oxidants 22 Enzymes 23 Chemical Substituents ~/-Radiation 25 Micromolecules 25 Homogenization 27

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V. Volume and the Theta Condition Vl. Summary 27

27

vi

Contents

CHAPTER 2

The Polysaccharide- Water Interface I. Introduction 29 II. Properties of Water 31 A. The Dielectric Constant B. Ionization 3I C. Activity 32 D. Specific Heat 34 E. Contraction and Expansion F. Surface Tension 34

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III. Polysaccharide-Water Interactions 3s IV. Influences on Polysaccharide-Water Interactions A. Bonding 36 B. Branching 37 C. Ionizing Groups 37 D. Heterogeneity 3'7 V. Polysaccharides as Adsorbents VI. Polysaccharides as Adsorbates VII. Summary 40

36

38 38

CHAPTER 3

State- and Path-Dependent Properties I. Introduction 41 II. Mass-Volume-Pressure-Temperature Relationships III. Electrostatics and Electrokinetics 42 A. Nonionic Polysaccharides 42 B. Ionic Polysaccharides 43 IV. Thermodynamics 47 A. Enthalpy 48 B. Entropy 49 C. Free Energy of Mixing 49 D. Irreversible Thermodynamics V. Kinetics 51 A. Diffusion 5I B. Order of Reactions

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52

VI. Hydrodynamics 53 A. The Imaginary Shear Plane 53 B. The Equivalent Hydrodynamic Sphere

53

41

Contents

VII. VIII. IX. X.

vii

Free Volume 54 Temperature Dependence Rheology 56 Variable-Path Processes A. B. C. D. E.

Sols, Gels, and Pastes Emulsions and Foams Xerogels and Films Aerosols 63 Suspensions 64

XI. Stability and Instability A. B. C. D. E.

s4 s9 60 62 62

64 66

Aging and Phase Separation Coacervation 67 Syneresis 67 Sedimentation 68 Encapsulation 68

XII. Summary

69

CHAPTER 4

Concentration Regimes and Mathematical Modeling I. Introduction 71 II. Concentration Regimes

71

A. The Dilute Regime 72 B. The Concentrated Regime C. The Semidilute Regime

III. Mathematical Modeling A. B. C. D. E. F. G. H. I. J.

73 73

74

The Stokes Equation 75 The Poiseuille Equation 75 The Huggins Equation 77 The Martin Equation 78 The Kraemer Equation 78 The Schulz-Blaschke Equation 78 The Newton Equation 79 The Power-Law E q u a t i o n 79 Hooke's Equation 80 The Activation Energy of Viscous Flow

84

IV. Size

85 A. The van't Hoff Equation 85 B. Light Scattering 86 C. The Contour and Persistence Lengths

90

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Contents

D. E. F. G. H.

The Mark-Houwink Equation The Hydrodynamic Volume Fractal Dimensionality 94 Sedimentation 95 Surface Area 98

V. Summary

92

91

t 00

CHAPTER 5

Additivity, Complementarity, and Synergism I. Introduction II. Interactions

101 101

A. Polysaccharide-Polysaccharide

B. C. D. E.

Polysaccharide-Lipid 105 Polysaccharide-Metal 107 Cyclodextrin and Amylose Clathrates Polysaccharide-Protein 108

III. Antagonism IV. Summary

102

108

113 i 13

CHAPTER 6

Thernml Processing I. II. III. IV. V. VI. VII.

Introduction 115 Atmospheric and Retort Processing 115 Low-Temperature Pyrolysis t 18 High-Temperature Pyrolysis 119 Maillard, Amadori, and Strecker Degradations Caramels 120 Summary t 2t

120

CHAPTER 7

Isolation, Purifwation, and Characterization I. Introduction

mz3

II. Extraction and Purification 123 III. Analysis 125 A. Detection of Charge and the Zeta Potential B. Functional Group Identity 127

126

Contents

IV.

V. VI. VII. VIII. IX. X. XI. Xll. XIII. XIV.

XV. XVI. XVII.

ix

C. Interaction and Conformation 128 D. Polydispersity 129 Molecular Weights and Sizes 130 A. Reducing End-Group Analysis 13 I B. Viscometry and Rheometry 132 C. Size Exclusion Chromatography 134 D. Membrane Osmometry 135 E. Light-Scattering Photometry 136 F. Sedimentation Equilibrium and Sedimentation Velocity Colorimetry and Spectrophotometry 137 CD and NMR Spectroscopy 140 Thermal Analysis t4t 144 Thermodynamic Variables Structural Elucidation 144 Volume Fraction 146 Hydrophilicity 146 Surface Area 147 Fiber 147 148 Pilot Plant Quality Control A. Identification 148 B, Aging 149 C. Sediment Volume 149 D. Syneresis 150 E. Consistometry 150 F. Texture 150 Polysaccharide Theta Conditions 151 Blending 155 Summary 15s

CHAPTER 8

C/ass/ficat/ons I. Introduction i 57 II. Chemical Classification 157 A. c~-D-Glucans 165 B. [3-D-Glycans 168 C. Fructans 173 D. Glycuronans 173 E. Sulfated Glycans 177

III. Summary

179

137

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Contents

CHAPTER 9

Saccharides in Fat Replacement I. Introduction

181

A. Hemicellulose B. Oligosaccharides

II. III. IV. V.

18 I

182

Isolation 183 Reactivity 184 Uses 184 Fat and Fat Replacement A. Essential Roles in Food B. Carbohydrate Fat Mimetics

VI. Summary

Appendices Appendix Appendix Appendix Appendix Appendix Appendix

I. 2. 3. 4. 5. 6.

186

187

188

Unit of Viscosity 189 The Schulz-Blaschke Equation i89 The Maxwell Model 19o Unit of TI/G 190 The Voigt-Kelvin Model 191 The Mark-Houwink Equation and t h e Hydrodynamic Volume 191

References Index

185

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223

Preface

Polysaccharides are continuing subjects of heightened interest as feedstock, health and dietary adjuncts, environmentally harmless additives, and alternatives for industrial commodities of dwindling supply or prohibitive cost. Because the food industry is arguably the largest consumer of polysaccharides, this book discourses the technology and processing of polysaccharides as they apply to food. One objective is to cultivate a fuller understanding of their responses to extrinsic stimuli, akin to those of synthetic polymers, whose study over the past 50 years has generated reams of theory. In addition, explanations of the response of small molecules need to be extended to polysaccharides, because, as Schwartzberg and Hartel (1992) succinctly stated, the physical chemistry of macromolecules applied to foods has been neglected (in the United States) until recently. In an oblique way, polysaccharides reemerged as a subject of advanced study when the importance of conformations and glass transitions in foodproduct development was recognized. Subsequently, applied texts on biological polymers were printed. By focusing on food polysaccharides, this author undertook an easy assignment, inasmuch as there was only one solvent, water, to consider. Moreover, Nature and the industrial supplier, whether inadvertently or deliberately, fix the inherent properties of these polymer molecules before they reach the processor and consumer. This text should be equally comprehensible to the student, researcher, plant manager, and layman with merely a modest technical background: It argues a common phenomenology of polysaccharides, albeit under different combinations of stimuli: It is intended to be a portal to elementary comprehension of everyday principles that are borrowed overwhelmingly from synthetic polymer science, and not just another assemblage of polysaccharide facts. The prose is supported by empirical and theoretical equations whose origins were not always heretofore identified to food technologists. The author expresses his gratitude and thanks to Professor Paul Okechukwu of the Federal Polytechnic Institute, Oko, Anambra State, Nigeria, and to Professor M. A. Rao of Cornell University for their expert review of the sections on rheology. The assistance of the following participants is

xi

xii

Preface

also gratefully acknowledged: Professor Y. D. Hang of Cornell University for his review of the section on enzymes; Ms. Roberta Wertman of the Bryn Mawr class of 1999 and Mr. Steve Comella of the University of Rochester class of 1996 for their magnificent assistance while summer interns in this laboratory. Special thanks go to Ms. Elaine Gotham of the Cornell University graphic arts department for her splendid reproductions and to Mr. Joe Ogrodnick and staff for their superb photographic contributions.

Reginald H. Walter

Symbols and Abbreviations*

A Asp A2 ao

aw ax of

C Cp, v s s s s s Co s s s s

CMC •

D Do Da D DE DP DS d

do di

d~

area specific surface area second virial coefficient area per adsorption site occupied by a single molecule water activity cross-sectional area of adsorbent expansion factor slope capacitance heat capacity concentration residual concentration concentration of the i th component molar concentration surface saturation concentration concentration of solvent (water) unfilled-sites concentration concentration at position or time 0 concentration at position or time 1 amount diffusing critical micelle concentration carboxymethylcellulose Flory-Huggins interaction parameter fractal dimensionality dielectric constant of water Daltons diffusion coefficient (diffusivity) degree of esterification degree of polymerization degree of substitution differential change density of solvent density of dispersion containing component i density of sphere

*Defined by context of use.

*oe Xlll

xiv

Symbols and Abbreviations

dn/dc i 0 A E Ea #

.q "l]a

~E "qo T]i

F f

f' f"

L G G' Gn

G* G g(t)

H

I

Io I, i is

K~ K

Kp k

kJ L l A

M M Mn Mw

refractive index increment infinitesimal change angle in degrees integral change energy, enthalpy apparent activation energy of viscous flow base of natural logarithm (2.718) strain strain rate coefficient of viscosity (viscosity) apparent viscosity elongational viscosity solvent viscosity viscosity of dispersion containing component i complex viscosity intrinsic viscosity under theta conditions force function of segment factor degree of carboxylation frictional coefficient Gibbs free energy storage modulus loss modulus complex modulus modulus time correlation function shear shear rate hydrophilicity scattered light intensity incident light intensity scattered light intensity at angle component i ionic strength ionization constant consistency partition coefficient constant (generic to a specific equation) kilojoule liter length dimension conductivi W wavelength wavelength of incident light monomolecular weight average molecular weight number-average molecular weight viscosity-average molecular weight weight-average molecular weight

Symbols and Abbreviations m i

P~o N

N na ni

n o

np n i n o nl n 2

n nm 1)

O o

12 co

P

P pI

Po po pH*

+o (~i, gel

ii(~,) pi

Q R Rg

Rh Ro R~,

xv

mass of component i chemical potential of the i th component in solution solvent equilibrium chemical potential above a solution solvent equilibrium chemical potential above pure water (standard chemical potential) Avogadro's number (6.02 • 102~ mol- 1) Newton number of adsorption sites number of moles of component i; refractive index of sol containing component i number of moles of solvent; solvent refractive index number of particles mole fraction of component i mole fraction of solvent molar concentration in phase 1 molar concentration in phase 2 refractive index nanometer (m • lO-9);.called millimicron (mix) in non-SI terminology exponent scattering wave vector solvent (water) electrophoretic mobility frequency property of a polysaccharide pressure isoelectric point solvent equilibrium vapor pressure above a solution solvent equilibrium vapor pressure above pure water (standard vapor pressure) pH at 50% neutralization volume fraction of component i volume fraction of solvent v o l u m e fraction of solute in a gel shape-dependent partical factor at angle 3.142 osmotic pressure angle angular velocity charge gas constant (8.317 • 10 -7 erg mo1-1 K -1", 0.08206 L atm K -1 mo1-1 radius of gyration hydrodynamic radius radius of gyration under 0 conditions Rayleigh ratio at angle t~ radius average radius average radius of a particle under 0 conditions root mean square end-to-end distance

xvi

Symbols and Abbreviations

S Sv s or

tro Cro,i

E t

tc to.5 t1 to ti T

T~

T, Tgel

Tm .f T TO

0

V, v

Vi Vo Vo V~ Kp

v, vo v~ ~c ~e

~)ex

vl ~)i ~)0 7)o T)m ~sp ~)0 ~)o w

unperturbed radius entropy sedimentation constant in Svedberg units (10-13 S) second surface tension surface tension of component i surface tension of solvent (water) interfacial tension sum of time correlation (decay) time half-life relaxation time flow time of a solvent flow time of a dispersion containing component i absolute temperature (degrees kelvin) cloud point (critical solution temperature) glass transition temperature gelation temperature gelatinization temperature melting temperature turbidity stress yield point the theta state; the theta condition (temperature a n d / o r solvent) volume elution volume volume of dispersion containing component i initial volume; void volume volume of solvent (water) volume of solvent in a column's micropores specific volume total volume volume under 0 conditions final volume of dispersion coil volume partial specific volume of electrolyte excluded volume free volume partial specific volume partial molal volume of component i void volume partial molal volume of solvent molar volume (volume of 1 mol of i) specific volume coil volume under 0 conditions partial specific volume of solvent weight

Symbols and Abbreviations Wi

X Xn X XO X1

Z

r

weight of a hydrocolloidal particle (i) weight percentage of solute DP function distance initial distance at 0 time (t = O) final distance (t = 1) interaction potential dissymmetry ratio zeta potential asymptotically equal to approximately equal to

oo

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CHAPTER I

Origin and Characteristics of Polysaccharides I. I n t r o d u c t i o n Polysaccharides are a class of biopolymers constituted with simple sugar monomers. Those used in commerce and industry are isolates from terrestrial and marine plants or are principally the exogenous metabolites of some bacteria; many are modified by partial organic synthesis, and a few are the product of total biochemical synthesis. Isolates from the same species, but from different culfivars, are remarkably chemically uniform (Jones and Smith, 1949). These so-called natural gums and mucilages have teleological significance in plant metabolism and function; one primary responsibility attributed to many of them is winter and drought hardinessma consequence of their water-binding characteristics. When extracted and purified, they are a major food item that is universally recognized as safe for human consumption. They are additionally an important industrial, scientific, and medical commodity. In petroleum recovery, mixtures of polysaccharides and sand are p u m p e d into oil-well crevices to provide transport channels for oil and gas. In science, polysaccharides are crosslinked for improved mechanical strength and acid, heat, and shear resistance, for use as adsorbents and ion exchangers. Glycotechnology is a currently active area of pharmaceutical and medical research on oligosaccharides for drugs. Cellulose, the most abundant polysaccharide, is the structural component of plant tissues; starch is the energy c o m p o u n d stored predominantly in seeds and tubers; glycogen is the animal counterpart of starch, but with shorter, more numerous branches. Cellulose and starch cohabit plant tissues with hemicellulose, protoplasm, lipid, and mineral matter in an organization interrupted by intercellular spaces that can amount to more than 50% of the total volume of some fruits and vegetables. A number of useful polysaccharides and their origins are listed in Table I.

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I. Origin and Characteristics of Polysaccharides

TABLE I Origin of Polysaccharides

Source Terrestrial plants Marine plants Bacteria Fungi Derivatization Synthesis Animals

Polysaccharides Starch, cellulose, inulin guar, karaya, pectin Agar, algin, carrageenan, furcellaran Xanthan, gellan, curdlan, dextran, cellulon Pullulan CarboxymethylceUulose(CMC), methylcellulose Cyclodextrins, polydextrose Glycogen, hyaluronic acid, chitin

In the food context, lettuce, apples, oranges, melons, cucumbers, etc., are uniquely succulent, turgid and crisp, because a small quantity of polysaccharide provides the coherence and mechanical strength necessary to embody 8 0 - 9 5 % water. Similarly, in vitro, fruit jellies are made possible with 3 5 - 5 0 % water. Polysaccharides perform numerous other functions in food: for example, in nongluten bread, tenacious polysaccharide systems retain CO 2 . Henderson (1988) ascribes the advantages of methylcellulose in the baking formula to a rise in surface tension at elevated temperatures, as a result of thermal gelation whereby the surrounding methylcellulose walls enclosing the gasfilled space are strengthened. The mechanical strength of gelatinized starch makes possible new structures from the extrusion-cooking of starchy influents, Dietetic beverages, sauces, etc., are often dilute dispersions in which a polysaccharide imparts "thickness" or " b o d y " to water. Syrups made with partial starch hydrolyzates compete with sucrose as sweeteners. In food technology circles, it is customary to hear about the excellent flavor-release qualifies of polysaccharide gels and their attenuating effect on the flavor intensity of others. There are distinct flavor-release differences between polysaccharides (Malkki et al., 1993) for which Baines and Morris (1989) offers the simple explanation that flavor molecules undergo a "restricted mixing" between the interior and the surface of a (polysaccharide) system--less so for r a n d o m conformations than for gels, due to an increased resistance to flow (viscosity) in the latter. Explained differently, the rate of diffusion of flavor molecules, f r o m the interior volume to the surface of a dispersion where they are sooner detected sensorially, accounts for the difference in flavor-release properties of polysaccharides. Gels hinder diffusion and sols facilitate it; consequently, nongelling polysaccharides are more efficient flavor releasers than are gelling polysaccharides. The same sensory intensity of flavoring and sweetening substances requires a higher concentra-

II. Physical-Chemical State

3

tion in a more viscous system than in a less viscous system (Morris, 1987). Unlike many monosaccharides of which they are composed, polysaccharides do not give the sensation of sweetness. The texture described as mealiness in fruit and vegetable products is imputed to pectin dissolution and migration from the middle lamella and the consequent separation of cells that then act as discrete units. The slippery feeling of some starchy foods is due to amylose leached from microfibrillar bundles in spherulitic granules during the first stages of water transport of molecular starch from the granules under the influence of heat (gelatinization). Human sensory responses to polysaccharides are largely a reaction to geometrical (size, shape, volume, surface area), optical (color, gloss), thermal (specific heat capacity, conductivity, diffusivity), electrical (conductivity, dielectric effect), and mechanical (stress) effects (Szczesniak, 1983). Polysaccharides are not always beneficial to humans: for example, lowmolecular-weight carrageenans are possibly toxic (Engster and Abraham, 1976); in sugar manufacture, crystallization inefficiency and fouling of filters are problems caused by dextran.

II. P h y s i c a l - C h e m i c a l

State

Fruits and vegetables in vivo are nothing more than living macromolecular systems or biocolloidal dispersions. Outside the living tissue, they are multicomponent dispersions and suspensions. It is these physical forms, influenced by one or more critical variables, that have many applications. The kind and scope of the polysaccharide response to stimuli in vitro depend on the polysaccharide's chemistry, the intensive properties engineered into it as a result of extraction, purification, and modification, and its interaction with the solvent surroundings. Many polysaccharides are at the same time polyalcohols, polyacids, and polyesters, composed of a topologically linear, main sequence of connected monomers in one of two anomeric shapes (chair o r boat), multiply configured, with varying amounts of methyl, acetyl, pyruvyl, etc., substituents, and occasional branching at regular and irregular intervals. The most stable ring form is equatorial (Thompson, 1992). The main sequence of monomers may consist of repeating di-, tri-, and oligosaccharide units, where each prefix denotes the number of simple sugars that constitute a unit. The dominant sugar determines its classification as a fructan, glucan, galactan, mannan, glycuronan, etc. A homoglycan contains a preponderance of one anhydrofuranosyl or anhydropyranosyl monomer; a heteroglycan has significant content of more than one monomer. Appendages to the fifth carbon of the pyranose ring lie outside the ring and consequently enjoy unlimited freedom

4

I. Origin and Characteristics of Polysaccharides

of rotation. The two secondary and one primary hydroxyl groups on the m o n o m e r ring offer opportunities for chemical substitution, with the objective of achieving altered, sometimes exceptional, behavior depending on the degree of substitution (DS). In polysaccharides, complete substitution (DS = 3.0) is difficult to achieve. Derivatized polysaccharides are commonly ethers and esters. As a result of pendant or ring ionizable groups, polysaccharides are largely divisible into two broad classes--ionic and neutral: starch and cellulose are typical of the neutral group. The natural ionic groups in polysaccharides are uronic, sulfuric, and phosphoric acid groups occurring as mixed salts of alkali and alkali earth metal ions (Na+, Ca 2+, Mg 2§ or methylated to varying degrees of esterification (DE). The DE may be given as a ratio of esterified (mostly methylated) C-6 carboxyl groups and the average derivatization sites per m o n o m e r or as a percentage of the total number of carboxyl groups in the molecule. Polysaccharide polyanions in the presence of excess cations, including H 3 0 +, simulate some properties of neutral polysaccharides. Neutral and ionic polysaccharides are amphiphilic (amphipathic) molecules whose functions (e.g., surface tension, density, water affinity) are dictated by their constitutive nature (chemical composition, the arrangement of atoms and molecules), extensive properties (e.g., volume, weight, energy content, viscosity, concentration), and solvent surroundings. The chemical definition of "constitutive" differs from the engineering definition that relates stress and deformation in governing equations. Constitutive properties are intensive properties, i.e., they are invariant functions of mass, whereas extensive properties rely on and change with mass. Properties that depend on the number of discrete units into which a solute has been subdivided are colligative properties. The distribution of substituents and branches in polysaccharides may be random or uniform. Distributions are isotactic when the substituents are all on the same side of the main axis, syndiotactic when they alternate on either side, and atactic when they are located at random. Regularity in the first two tacticities is conducive to crystallization (Sperling, 1986). Positional isomerism can lead to dissimilar properties, e.g., sulfate in carrageenans influencing their ability to gel (Anderson et al., 1968; Guiseley et al., 1980).

A. Molecular Weight and Degree of Polymerization The molecular weight ( M ) of a polysaccharide is the gram mole or molar mass of 6.023 • 102~ molecules (Avogadro's number) that ideally are of a single size. Extracted, isolated, and purified polysaccharides in the same class, from the same sampling source, are seldom uniform in shape and size and are therefore preferably characterized by an average molecular weight M.

II. Physical-Chemical State

5

M and M are experimental quantities that are arguably less than those of the parent polymer, in vivo, and vary with the method of measurement, most of which involve expensive apparati. Depending on the analytical technique, either quantity may be a weight-average (Mw), number-average (M,), or viscosity-average (M v) property. Methodologically, M w > M v > M--,, each capable of differing by a factor of 1-2 X l02 (Tanford, 1961; Walter and Matias, 1991). All the methodologies have their advantages and disadvantages. The high number of monomers constituting M and M is referred to as the degree o f polymerization (DP). Being predetermined by the manufacturer's methodologies, the average DP is constant but widely distributed from the mean, and is thus unsuitable without fractionation for most experiments i__nvolving purchased samples. M and M are not related only to mass; they are equally a function of charge, solvent activity, and concentration. Somegeneralizations can nevertheless be made about the DP: the lowest DP stimulates the highest kinetic activity at the same weight concentration; low DP polysaccharides are more "soluble" and reactive than high DP polysaccharides; a high DP is necessary for dispersion viscosity measurably above that of water. M a n d M are alternatively characterized by the radius of gyration (Rg), which is visualized as the radius of a thin circle transversely excised from an imaginary molecular cylinder, having a proximalend fixed at the center and a distal end traveling randomly along the circumference. The mass density is highest at the proximal end (Tanford, 1961). Random-walk theory indicates that the distal end will eventually maintain an equilibrium distance in the vicinity of the proximal end. Unlike simple reducing sugars and oligomers, polysaccharide s do not normally reduce Fehling's and Benedict's solutions--the historical reagents for identifying reducing activity (Cu 2+ to Cu I + after boiling for 5 s and left standing). The polysaccharides and higher DP oligosaccharides are ordinarily chemically inert, relative to their corresponding monomers and repeating units, because the high ratio of monomers to reducing end groups masks the carbonyl function. For example, assuming amylose _M= 104 Daltons (Da) and amylopectin M = 106 Da, these starch fractions each contain one reducing end for every 55.6 and 5560 monomers, respectively (assuming exclusively 1,4-~ linkages). A Da is the unit of molecular weight equivalent to one-sixteenth the gram atomic weight of oxygen. The importance of M is unmistakable when it is recalled that 180 g glucose in 103 g water [1 molal (m)] lowers the freezing point by 1.86~ whereas the same weight of polysaccharide (1.80 • 10 -3 m, assuming M = 105 g) has no such effect and, moreover, it is difficult if not impossible to disperse 105 g of any polysaccharide in 103 g water. Identical arguments hold for boiling-point elevation and osmotic pressure. However, polysaccharides at much lower concentrations exercise influences, most prominently by

6

I. Origin and Characteristics of Polysaccharides

structuring water into high-viscosity fluids and obstructing the crystal order at and below 0~

B. Configurations and Conformations The earliest studies of natural polymers, predating those of synthetic polymers, elaborated an association hypothesis of a primary macromolecular structure held together by physical bonds (Purves, 1943). It was later proven that the primary bonding is covalent and that a tertiary structure results from physical bonding between primary structures that are themselves highmolecular-weight compounds. Subsequently, "well-defined generalizations" emerged as the foundation of a n e w discipline--polymer science (Flory, 1953). Advances in synthetic polymer research have outpaced those in biopolymer research. The primary structure of a polysaccharide is the main sequence of connecting sugar monomers covalendy linked as oL and [3 glycosides. The constitutively fixed bond lengths and angles controlling the ring orientations comprise a secondary structure (configuration) whose effects are most acute in heterobiopolymers like pectin and carrageenan, for example, wherein rhamnose in the "hairy" regions of the former (Thibault etal., 1991) and sulfate in the latter (Stanley, 1990) cause kinking. Interchanges of atoms in the primary and secondary structures can occur only by rupture and reformation of valence bonds. The single-bondedness of the glycoside linkages enables different segments to rotate independently in solid, liquid, and gaseous space in the manner of a freely jointed chain, unlike those of synthetic polymers where there may be bond and steric hindrances:. Independent segment rotations culminate in a tertiary structure (conformation). Polysaccharide quaternary structures (Fishman et al., 1991; Dea et al., 1972) consist of clusters of tertiary structures held together by a net s u m of intermolecular forces (hydrophilic, electrostatic, ionic, and hydrophobic) under various influences of solute concentration, solvent, electrolytes, temperature, and shear. Tertiary and quaternary structures are physically reversible. The DP exerts much influence on conformation . A low DP is conducive to the development of rodlike geometries and crYStals, whereas a high DP is conducive to random-coil and amorphous behavior. The highest incidence of coiling is found in the amorphous regions in linear polymers (Sperling, 1986). Hydrophilic, electrostatic, and ionic forces are dominant in polysaccharide systems, given their constitution; less dominant is t h e hydrophobic force that is more likely to be encountered in proteins and other biochemical multisubunit particles (Ben-Naim, 1980). Linearity of the polysaccharide primary chains facilitates parallel ordering of the tertiary structures in two (e.g., [3 sheets of cellulose) and three 9

II. Physical-Chemical State

7

(e.g., helices in xanthan gum) dimensions that grow in time to floccules. 1 The growth of these supramolecular associations (Finklemann and Jahns, 1989) is accelerated or retarded by environmental stimuli. FlocculesBnot single moleculesBdominate macromolecular activity above a critical solute concentration. In the words of Doi and Edwards (1986), "the macroscopic properties (of a polymer liquid) depend only on a few parameters specifying the molecular characteristics, and insofar as these parameters are the same, different systems behave in the same way." Parallelism in localized regions of polysaccharides creates three-dimensional order between segments of the primary chain that is conducive to crystallite formation, which in turn has a profound effect on the polysaccharide response to ambient stimuli. A high incidence of crystallinity spawns refractoriness; a low incidence contributes to amorphism. X-ray diffraction of cellulose has shown that oxygen-oxygen distances in one direction of the tertiary structure (the unit cell) are shorter than the van der Waals diameter of oxygen atoms in an organic micromolecule. This suggests strong lateral forces (Tanford, 1961) operating at these sites, accounting for refractoriness. A now virtually discarded fringe micelle model describes polymer crystals as consisting of ordered regions bonding in series with amorphous regions in the same molecule (Cowie, 1991). Contemporary theory favors crystallite formation from single molecules acting as crosslinks between other linear molecules (Severs, 1962). Natural and artificial events (e.g., branching, kinking, bulky substitution, high cosolute additions) disrupt the tendency toward parallelism, crystallite formation, and refractoriness. Supramolecular assemblies assume smectic, cholesteric or nematic orientations, whereby the planar axis may be perpendicular (smectic) or parallel (cholesteric) to the molecular axis. In a nematic orientation, the assemblies are not planar, although the molecular axes are parallel (Elias, 1979). Parallelism and stereoregularity are in harmony with the kind of molecular order that ends in helix as well as crystallite formation (Billmeyer, 1984; Cowie, 1991; Rinaudo, 1992) in amylose (1,4-e~), cellulose (1,4-[3), and curdlan (1,3-[3), for e x a m p l e ~ t h r e e stereoregular polysaccharides t h a t do seem to favor these conformations. Deesterified carboxyl sequences in the primary structure of low-methoxyl pectin and between poly-L-guluronate sequences in alginate facilitate bridging by Ca 2+, illustrated in the "egg-box" model of gelation (Rees et al., 1982). Rinaudo (1988) interprets the effect of this blockwise deesterification on pectin gelation to mean a requirement of a critical, minimum length of a continuous series of carboxyl groups. Rhamnogalacturonan blocks are not organized into the conformation prerequisite to gelation (De Vries et al., 1981), because kinking (Rees and Wright, 1971; Oakenfull, 1991) precludes the parallel orientation. Guar and locust bean gums are gelling and nongelling galactomannans, respectively; the former 1. Flocculation, coalescence, coagulation, and aggregation are herein used synonymously.

8

I. Origin and Characteristics of Polysaccharides

has more extensive, uniformly spaced substituents. In cold water, guar gum hydrates more than locust bean gum, in an apparent contradiction of the expectation of regularity in the fine structure. As exemplified by the hydratability of these two gums, polysaccharide properties are integrals of many factors. Polymers are modeled on three idealized conformations, viz., a solid sphere (Harding et al., 1991a), a random coil, and a stiff rod (Doi and Edwards, 1986). Linear polymers are more or less randomly coiled in solution (Eisenberg and King, 1977); so are polysaccharides (Morris, 1976), inasmuch as the freely jointed chain offers them endless possibilities for rotation averaging conformations within the extreme bounds of a flexible coil and a rigid rod. Computer modeling suggests an improbability of very short and very long distances between proximal and distal groups; most fall instead at intermediate distances from each other. The almost infinite number of possible segment associations must be treated statistically (Smith, 1982). It is the thermodynamics of the particular system that dictates whether polymer molecules acquire the habit of a chain or rod (Rinaudo, 1988). When there is interaction, the resulting conformation may not necessarily acquire a state of minimum energy (Tvaroska et al., 1992). The study of Tvaroska et al. (1992) conveyed the picture of paired molecules (of K-carrageenan and mannan), approaching each other in their respective ground states, changing conformation, and maximizing the van der Waals and Coulombic attractions. Solvent conditions have a profound influence on conformation, as proven by glutamic acid, a random coil at high pH, but a helix in acidic media where ionization is depressed (Doty et al., 1957), and by hyaluronic acid, a random coil in dilute solution at neutral pH but very stiff in acid (Shah and Barnett, 1992). A perfect helix can exist as a random coil at a sufficiently high DP (Elias, 1979). According to Alfrey et al. (1942), a solvent manifests i.ts effects through an unbiased, mean statistical configuration that shifts between a completely curled (in an energetically unfavorable solvent) and a completely extended (energetically favorable solvent) geometry at constant temperature. An energetically unfavorable solvent is described as "poor," and an energetically favorable solvent as "good." A good polysaccharide solvent maximizes and retains an expanded conformation, as a result of solute-solvent interactions, at the expense of solute-solute interactions: a poor solvent shrinks the molecular volume and retains the polysaccharide as a compact coil. Water, occasionally modified by a small volume of ethanol and electrolytes, is the universal food solvent. In this medium, a polysaccharide polyanion's molar volume (v m) is maximum, because charge repulsion forces the molecules into the stiff (uncurled) conformation of a rigid rod that cannot be extended further, but can be oriented (Odell, 1989). v m is larger for extended, stiff conformations than for compact random-coil conformations (Berth, 1992). As solvent quality deteriorates, e.g., by addition of salt or

II. Physical-Chemical State

9

ethanol, molecular motion declines, •m becomes less, and the solute molecules begin to exercise a strong attraction for each other. The condition determined by temperature and solvent that allows neither polymerpolymer nor polymer-solvent attraction is the theta (0) temperature and 0 solvent, respectively. A nonsolvent is any organic, water-miscible liquid (e.g., ethanol and acetone) that inclines a dispersed solute toward the 0 state. The rigidity (or flexibility) of dispersed polyanions is basically a function of the solvent's ionic strength through its influence on electrostatic repulsion (Eisenberg and King, 1977). Hence, a polyanion is most rigid in pure water where, at any instant, the fixed negative charges foster an equilibrium segmental and counterion distance at its farthest. Excess cations relax the rigidity by shielding the charged sites, thereby allowing the primary chain to revert to a flexible coil (Miles et al., 1983). Neutral polysaccharides experience no charge restriction and therefore freely migrate toward and away from each other in dilute solution. Stiffness can cause polysaccharide polyanions to appear as pseudocrystals (Barnes et al., 1989), because they easily order themselves into an apparently crystalline solid at rest and revert to a liquid when agitated. The 1,4-[3-glycoside linkage is inherently stiff and extended (Blackwell, 1982): such molecules incur anisotropy. For a constant polysaccharide mass, an extended (random) coil exposes more surface area than does a helix, and a single helix exposes more than a double helix. The energy content of a polymer molecule is a property of its surface area. Thus, one consequence of a coil-to-helix transition is a diminution of the macromolecular exposed surface area and energy in compliance with the law of entropy. An increase in viscosity coincides with an increase in surface, inasmuch as the resistance to motion covers a wider area.

1. Specific Conformations

Depending on the DP, charge, and solvent conditions, CMC may be a random coil or rod (Guo and Gray, 1991). According to Mitchell and Blanshard (1979), short chains in alginate and pectate gels are stiff and extended, while those of higher DP are flexible. Other specific conformational descriptions of dispersed polysaccharides are random coils for konjac mannan (Kishida et al., 1978) and guar gum (Robinson et al., 1982), rods for gum ghatti (Srivastava and Rai, 1963; Elworthy and George, 1963), stiff coillike for starch (Banks and Greenwood, 1975), ribbon (Ring, 1982; White, 1982), stiff extended ribbonlike, and planar for cellulose (Glass, 1986) and chitin (Blackwell, 1982), extended ribbon for alginic acids (Atkins et al., 1970) and cellulose (Blackwell, 1982; Belitz and Grosch, 1987), stiff coil for agar (Hickson and Polson, 1968), short stiff spirals for gum arabic (Meer, 1980b), single helix to triple helix for curdlan (Marchessault and Deslandes, 1979; Stipanovic and Giammatteo, 1989; Ross-Murphy, 1991), double helix

I0

I. Origin and Characteristics of Polysaccharides

for carrageenans and agarose (Ross-Murphy, 1991), and left-handed double helix for agarose (Arnott et al., 1974). Veis and Eggenberger (1954) suggested the possibility that branching was responsible for the stiff arabic acid coil. The agarose gel model of Hayashi and Kanzaki (1987) contained alternating helix and kink segments. Some polysaccharide molecules, e.g., agarose (Hayashi and Kanzaki, 1987), hardly change dimension. Pectin conformation may be a strongly pleated ribbon (Belitz and Grosch, 1987) or a random coil (Axelos et al., 1987; Berth, 1988; Berth et al., 1990), but the data of Harding et al. (1991b) on a citrus pectin (approximately 70% anhydrogalacturonic acid in 0.03-M phosphate buffer, M = 2.0 X 104-2.0 X 105) ' fitted the model of a rod with some anomalous features indicative of a wormlike coil. The wormlike conformation is described as different orders of flexibility, from a random-walk chain to a stiff rod (Dautzenberg et al., 1994). Contemporaneously, Hourdet and Muller (1991) indicated that a flax pectin (80% anhydrogalacturonic acid in 0.2-M sodium chloride) in an inclusive molecule-weight distribution range behaved like an extended coil. Pectin random coils are further described as continuously mobile (Morris et al., 1981). Amylose has been described as a double helix (Gidley, 1989) and a random coil (Ring and Whittam, 1991). The prevailing hypothesis is that it is a flexible random coil of extended helical segments connected by deformed nonhelical segments (Szejtli, 1991). Gidley (1988) identified two molecular structures in an amylose g e l m o n e of relatively immobile double helices and the other of all energetically permissible conformations. Cellulose exists in helical conformation in the crystalline state (Guo and Gray, 1994). Double helices have been implicated in the gelation of many polysaccharides, e.g., agar, carrageenan (Rees, 1972a, b; Glass, 1986), and gellan (Chandrasekaran et al., 1988a). This last gum is proposed to exist in solution, under nongelling conditions, as a disordered coil at high temperatures and as a double helix at low temperatures (Robinson et al., 1991). Marine polysaccharides are often suggested to be in a double-helix conformation (A. H. Clark, 1992). Two forms of carrageenan (K and ~) are random coils above 50~ and double helices below (Anderson et al., 1969); these are the structural units that must then aggregate preceding gelation (Morris et al., 1980). The gelling mechanism of alginate has been deduced as an association of double helices made up of guluronate moieties that bind divalent ions cooperatively (Rees, 1969; Sime, 1990). Callet et al. (1988), unable to detect any disassembly of a xanthan double helix, explained xanthan rheological properties on the basis of preexisting, ordered conformations without change in molecular weight. Curdlan adopts a triple helix conformation in the dispersed and solid states (Deslandes et al., 1980) and reverts to a random coil in 0.25-M sodium hydroxide (Stipanovic and Giammatteo, 1989). Some glucans are ordered in dilute alkali and disordered at higher concentrations (Ogawa et al., 1972).

II. Physical-Chemical

State

I I

Braudo (1992) showed that helix formation is not a prerequisite to gelation: rather, gelation may result from hydrogen bonding and from intermolecular coordination binding of cations (e.g., calcium in the case of low-methoxyl pectin and potassium in the case of K-carrageenan): for example, xanthan is a random coil at high temperatures and a helix at low temperatures (Griffiths and Kennedy, 1988); upon heating, its dispersions undergo a change of state from an ordered helical (Rees, 1972a, b) to a disordered coil structure. The conformation is influenced by the degree of ionization--helical below 0.85 and random coil above (Young and Torres, 1989). In the native state, this gum has also been described as rigid and rodlike (Sandford, 1979) in single, fivefold helix conformation (Griffiths and Kennedy, 1988) that converts to a random coil at high temperatures in low-ionic-strength media (Morris et al., 1977). Upon cooling, the molecules revert to their native conformation and align themselves with unsubstituted regions in galactomannans (Dea and Morris, 1977) in a gelling mechanism of cooperative crosslinking. Xanthan gum was shown to be stiffer than CMC and alginate: all three are ionic polysaccharides, with CMC having slightly more flexibility than alginate under identical conditions (R. C. Clark, 1992). The invariant nature of xanthan dispersion properties is attributed to the stability of the tertiary structure. The indifference of this gum to salt is explained by its already rigid conformation (Morris, 1976). Concentration has a role in conformation: dextrins are random coils in dilute dispersions and compact coils in less dilute dispersions (McCurdy et al., 1994). 2. Interaction, Disorder-Order and Order-Disorder Transitions

In the absence of a formal definition, "interaction" is understood to involve reciprocal action between two polymer molecules: in the process, one or both undergo a transition from an ordered to a disordered state or vice versa. Gelatinization (Biliarderis, 1992; Kokini et al., 1992) is an order-disorder transition of starch granules interacting with water in a critical temperature range. The initial, intact spherulites have the organization of crystalline regions of relatively short, hexagonal fibrils surrounded by amorphous regions. In the first stage of the transition, the granules hydrate and swell; in the terminal stage, they lose rigidity, become disordered, and disintegrate. Finally, fluid starch exudes from the physical confines of the former granule into the outer volume of water. Complete gelatinization is irreversible. After a period of time (aging), the dispersed (disordered) amylose reverts to a differently ordered association (retrograded starch), whereby linear molecules and segments are returned to some semblance of an organized crystallite, albeit vastly dissimilar to the initial structure. Retrogradafion is irreversible in ordinary storage, but can be reversed by heat and moisture intervention.

12

I. Origin and Characteristics of Polysaccharides

The interaction between xanthan and konjac mannan gums in an aqueous sol leads to gels in an ordered conformation at 42~ and in a disordered conformation at 57~ (Williams et al., 1991). Thermoplastic gels are a reversible disorder-order transition and thermosetting gels are opposite and irreversible. The significance of all these conformational transitions is the dependent change in properties and function concurrent with a change of phase, location, size, a n d / o r energy status: the final phase, location, size, a n d / o r energy status is not identical to the initial. The order-disorder transition of a polysaccharide crystallite is loosely referred to as melting, notwithstanding the absence of the solid-liquid phase change, as in the melting of fat for which the word was reserved. Broken-curve heating (Hersom and Hulland, 1981) in soup and juice concentrates, wholegrain corn, starchy fluids, etc., results from an order-disorder transition whereby a solid or gellike mass is first heated by conduction and then by convection as the energy content and subsequent mobility of the mass increase.

C. Colloidal Activity By definition, a colloid has at least one molecular or macromolecular dimension in the 1-500-nm range (Jirgensons and Straumanis, 1962). Polysaccharides fit this definition. The nominal width is the orbital diameter in atomic proportion to the hydrated molecule rotating around its long axis. Heyn (1966) reported widths of 25-40 A (2.5-4.0 nm) for single cellulose microfibrils; Kanzawa et al. (1989) reported 50-250 nm for gelling polysaccharides and 10-20 nm for nongelling polysaccharides. Pfannemi~ller and Bauer-Carnap (1977) measured an average of 10 nm (100 A) for fibrils and fibrillar aggregates of amylose, DP = 100-7200. According to Harada et al. (1991), microfibrils in curdlan gels comprising subunits 2 - 3 nm wide are themselves 20-25 nm wide, Crystallites approximate a length of 46 nm and a diameter of 7.3 nm (Cowie, 1991). Cellulose microfibrils can be 10-103 nm long (Weibel, 1994). Fishman et al. (1986) determined length-to-width ratios of 120-200 per pectin. Polymers, organic and inorganic molecular clusters are synonymously called macromolecules to distinguish them from the smaller sizes of organic acids, salts, ethanol, etc. (micromolecules). Properties unique to organic macromolecules begin to be manifest at M = 103-106 Da (Billmeyer, 1984; Sperling, 1986). 1. Dispersibility

Polysaccharides are not soluble in water in the classical sense that micromolecules (e.g., sodium chloride and sucrose) are. In micromolecular solutions, solute and solvent are indistinguishable to the solubility limit, and

II. Physical-Chemical State

|3

3.51

d

3.0

2.5

o~

2.0

0

1.5 1.0 0.5

0 0

10

20

30

40

50

60

70

80

90 100

Time (hr)

Figure I Dispersibilityof 50 g pectin in l0 s mL water in a 2-L two-neck distilling flask held at 26~ with mild stirring. The weight per volume percentage concentration was calculated, after periodically withdrawing 10-mL samples from the suspension and oven-drying them to constant weight.

the solutions are described as h o m o g e n e o u s , because the two c o m p o n e n t s m e r g e into a m o n o p h a s e . Polysaccharide-water systems in any solute-solvent ratio do not have a solubility limit; instead, they are h e t e r o g e n e o u s dispersions called sols, possessing an interface, albeit imperceptible at times. Polysaccharide sols may, however, exhibit a pseudosolubility limit where, for every molecule diffusing out of the solvent phase, one enters it, until the system is disturbed and m o r e solute leaves the liquid phase than enters it, or solute begins to be deposited on existing dispersed solute to the point of phase inversion (i.e., a solid-in-liquid to a liquid-in-solid transition). In the latter, the rate of diffusion is initially fast and then slows as the pseudosolubility limit is a p p r o a c h e d (Fig. 1). From the metastable pseudosolubility limit, precipitation is possible, given the appropriate stimulus.

2. Hydrophilicity Polysaccharides, r e m a r k a b l e for their water affinity, can e m b o d y m a n y times their weight or volume of this s o l v e n t m h e n c e the n a m e hydrocolloids. Polysaccharide dispersions are either hydrosols, hydrogels, or xerogels (de-

14

I.

Origin

and

Characteristics

of Polysaccharides

hydrated hydrogels). The high rate and capacity of absorption mimic swelling that is characterized by a swelling ratio defined as the quotient of the volume of dispersion containing component i (V/) at maximum hydration and the initial volume (V0) of a xerogel. The absorbed water is customarily referred to as water of hydration: it has a lubricating and plasticizing effect on human ingestion. Bulkiness and the feeling of satiety in the human regimen, derived from consumption of fibrous fruits and vegetables, are sensory reactions to a large V i / V o. Hydrated tissues are more digestible than dehydrated tissues, because of the softened texture resulting from water of hydration. Polysaccharide polyanions swell to many times their volume at 0 (V0), more t h a n do neutral polysaccharides, and shrink proportionately: V i / V o is markedly enhanced by water and depressed by nonsolvents and cations through their action on Vii. Moisture is lost linearly from dehydrating plant tissues to 20-30% (Fig. 2), indicating that some water of hydration is loosely held. Apparently

100. ~ , 90.0 - ,;

z

80.0 -

",~

_z

.

< 70.0 u.i Lu rr

60.0

1-

5o.o

0

40.0

0

30.0

o~

"

20.0 10.0~ =

~RAPE -

00.0 I

0

.

,

I

1

.....

APPLE L

2

TIME (HR)

J

3

F i g u r e 2 Drying curves for grape and apple pomace held at 1050C' The percentage moisture remaining was calculated, after withdrawals of 10g samples from a 10~-g batch and drying them at the designated intervals.

II. Physical-Chemical State

15

dehydrated tissues can retain single-digit moisture levels, while appearing to be completely dry. This property is exploited in the use of polysaccharide xerogels as humectants, e.g., anhydrous starch added to baking powder to retard caking.

3. Surface Area and Surface Tension A dispersed hydrocolloid exposes an inordinately large surface area relative to its volume or weight. The relationship between energy (E) and the total exposed surface (A) is stated as

E=o'A.

(1.1)

is the surface tension of the hydrocolloid in an energy per area unit. As a result of or, the surface molecules are attracted toward the interior of a substance: the larger the value of ~r, the stronger is the internal cohesion and the less is the surface area. A dispersed polysaccharide spontaneously seeks to lessen A, in compliance with the second law of thermodynamics pertaining to energy minimization. It follows from Eq. (1.1) that fine subdivision of a polysaccharide solute requires energy: the corollary is that the more surface a dispersed polysaccharide exposes, the more energetic it is and, consequently, the less stable it should be. The interfacial tension (%,i) is the equivalent of a when the substance interfaces a substance other than air.

4. Viscosity, Elasticity, and Viscoelasticity The resistance that macromolecules encounter as they flow past each other in a solvent is called viscosity: the larger the surface area, the higher is the viscosity. Random coils above 0, exposing a larger surface area than other configurations, give a higher viscosity; compact r a n d o m coils offer less resistance to flow (lower viscosity) than extended rods. The energy expended during flow, less at high temperatures (Severs, 1962), is not recoverable. The viscosity created by infinitely thin parallel fluid planes transported co- or countercurrently is called simple shear viscosity. Planar flow may be initiated in other geometries, e.g., rotational, telescopic, and twisting (Van Wazer et al., 1963). Humans evaluate simple shear viscosity as the sensation of thickness. Certain fluids, e.g., honey and paint, are quite resistant to flow and are therefore considered to be very thick or viscous. Most polysaccharide dispersions increase in simple shear viscosity with the DP and concentration and decrease with increasing temperature. Viscous transport is laminar or streamline, if molecules in the (imaginary) planes of fluid in any geometry, flowing along a velocity gradient, do so with the same translational and rotational velocities. Streamlines are occa-

16

I. Origin and Characteristics of Polysaccharides

sionally visible as contours of thin batter traveling slowly around a spoon or fork during stirring. Complex flow patterns are encountered when the fluid contours are forced to change shape a n d / o r dimension suddenly, as, for example, during passage through porous media (Lapasin and Pricl, 1995). At high velocities, turbulent flow results when molecules in any one plane travel with different translational and rotational velocities. Streamline and turbulent flow are characterized by a Reynolds n u m b e r (2200), which is a dimensionless computation involving flow distances and time. Viscosity (~q) is streamline below 2200. An elongational or extensional viscosity (~qE) develops as a result of a conformational transition when disperse systems are forced through constrictions, or compressed or stretched (Kulicke and Haas, 1984; Rinaudo, 1988; Barnes et al., 1989; Odell et al., 1989; Clark, 1992). The intuitive logic is that the r a n d o m coils resist the initial distortion. ~]E is believed to elicit the h u m a n sensation of stringiness (Clark, 1995). If shear viscosity is denoted ~qi, rheologists define a Trouton ratio as ~qE/~i wherein ~qE > ~i by a factor approximating 3 for uniaxial extension and 6 for biaxial extension. Alternatively stated, the Newtonian ~qi calculates to one-third to one-sixth ~qE (Steffe, 1992). Barker and Grimson (1991) modeled the flow of deformable particles after a free-draining floc whose shape, orientation, and internal structure ranged between the extremes of an extended chain and a folded globule. They interpreted the u n h i n d e r e d motions of free-flowing, deformable droplets to result from an unbalanced force imposed by the flow field, resulting in rotations around the particles' center of mass; this rotation is superimposed on the steady translational motion. If a fluid mass merely deforms under slight stress and the deformation (strain e) is completely recoverable spontaneously u p o n removal of the stress, the fluid is an elastic fluid. Elasticity is sensitive to temperature and relatively insensitive to concentration, because of the already high solute content. Fluids exhibiting stages of viscosity and elasticity t h r o u g h o u t the flow continuum are viscoelastic fluids. There are five transitional stages of viscoelasticity, viz., the glassy, leathery, rubbery, rubbery flow, and viscous stages (Cowie, 1991).

5. Light Scattering and Turbidity The cloudy appearance of a dispersion seen when a beam of light impinges on it is the result of light scattering. Cloudiness is evidence that a boundary indeed exists between the liquid and solid phases. The conical rays observed as light emerges from a small aperture into dimly lit s p a c e m n a r r o w at the point of incidence a n d wider away from i t - - a r e the well-known Tyndall effect. Light scattering is a function of colloidal particle size. Diameters less than 0.05 nm obey Rayleigh's law, stating an inverse mathematical

II. Physical-Chemical State

17

relationship of wavelength (R) to turbidity in isotropic solutions. Particles that have at least one dimension equal to or greater than R undergo Debye scattering in which an angular dependence of the wave intensity is taken into consideration (Oster, 1960). Scattering is inversely proportional to the fourth power of the incident wave frequency: hence, dispersed polysaccharides scatter shorter )~ much more than they do the longer )~, and violet waves are scattered 16 times as strongly as red waves (Smith and Cooper, 1957). Optically anisotropic polysaccharides scatter light more intensely at forward angles ( < 90 ~ than at backward angles (> 90~ Scattering is also a function of concentration of the dispersed phase.

6. Optical Activity and Anisotropy Sugar monomers contain asymmetric carbon atoms and are therefore optically active; i.e., they rotate plane polarized light. Optical activity diminishes with polymerization, and conversely increases with saccharification. The inherent asymmetry of polysaccharide molecules enables each one to be characterized by an axial ratio (ratio of cross-sectional and longitudinal distances normal to each other). Instrumentally, the relative intensities of light scattered at 45 ~ and 135 ~ have been rationalized into a dissymmetry coefficient (Z) capable of providing qualitative information about macromolecular dimensions (Stacey, 1956). Anisotropic molecules have axial ratio greater than 1. By phase microscopy, starch granules from different plant species can be identified through their distinctive polarization crosses.

7. Surfactancy and Protective Colloid Action Incompatible commingling molecules separate soon after the commingling stimulus is withdrawn: such systems have short lifetimes and are therefore said to be unstable. For longer life, surface active compounds (surfactants), efficacious in small quantities, are added to decrease the contact angle between the immiscible surfaces, lower or, and permit the interfusion of the immiscible surfaces. The stabilizing function of macromolecular surfactants in solid-liquid systems is exercised through protective colloid action. To be effective, they must have a strong solution affinity for hydrophobic and hydrophilic entities. In liquid-liquid systems, surfactants are more accurately called emulsifiers. The same stabilizing function is exercised in gas-liquid disperse systems where the surfactants are called foam stabilizers. Polysaccharides possess surfactancy to varying degrees. Methylcellulose is a very efficient protective colloid at 0.01% (Dow Chemical Co., 1990). Random coils (of xanthan) are more surface active than helices (Young and Torres, 1989).

18

I. Origin and Characteristics of Polysaccharides

D. Heterogeneity and Homogeneity Isolated, purified polysaccharides are beset with numerous heterogeneities. To begin, structural and positional isomerism are caused by different locations and distributions of rhamnose, sulfate, and acetyl groups, for example, in the primary structure. Fine structures in galactomannans and pectins, consisting of smooth and hairy regions where proteins interact (Kravtchenko et al., 1993), are sites of microheterogeneity. There is normally a wide spectrum of DP that arises from the use of chemical reagents in extraction processes. DP heterogeneity was denoted by Elias (1979) as polymolecularity and by Everett (1988) as polydispersity. The designation M refers to a monomolecular or monodisperse polymer, and M refers to a polymolecular or polydisperse polymer. Dimensional heterogeneity is typical of micelles and pores in a polysaccharide gel subjected to variations of the same physical treatment. Size-homogeneous fractions of a polysaccharide can be separated from a polymolecular sample by a variety of separation techniques. Washing is an elementary, unintentional homogenizing operation, because lower DP fractions ( < 10 ~ Da) are apt to remain in the wash. Homopolymers are characterized by sharply defined boundaries and discontinuities that contrast with the diffuse boundaries of heteropolymers in the separation display. In natural and artificial heteroglycan synthesis, the distribution of a copolymer or fine structure may be in random, sequential, or block design, e.g., the galactomannans. Rhamnogalacturonan regions in pectin alternate with homogalacturonan regions (Kravtchenko et al., 1993). Block copolymerization confers unusual bulk properties on linear polymers (Finkelmann and Jahns, 1989), and variations in copolymer composition alter the electrostatic environment and consequently the size, structure (Hunkeler et al., 1992), and properties of polymers. At moderate to high concentrations of polysaccharides, interactions over the length of the macromolecules beget an assortment of contact or junction zones (Rees, 1969) whose heterogeneous distribution is explained as follows (Silberberg, 1992): different segments are in different solubilities and hence different conformational states, with the result that some contacts are of longer duration than others. At high concentration, strong associations between the less soluble and more sterically complementary segments develop into crystallites with very long lifetimes. Junction-zone heterogeneity is widespread in gels. Fluctuation theory explaining light scattering in dilute dispersions supposes transient heterogeneities initiated by density variations (Stacey, 1956).

E. Polymorphism, Hysteresis, and Syneresis The polysaccharide primary structure affords numerous opportunities for hydroxyl-and carboxyl-group interactions, leading to polymorphism (al-

lit. Phenomenology

19

lomorphism) and hysteresis. The difference between polymorphs is in the crystal unit-cell structure. There are at least four crystalline forms of cellulose, based on different packing of the primary chain (Blackwell, 1982), and three forms of granular starch, based on the packing of double helices (Noel et al., 1993). The differences are largely in the unit-cell dimensions and the crystallization and precipitation temperatures. One form of starch, precipitated with alcohol, is in a symmetrical molecular arrangement and is readily dispersible in cold water (Kerr, 1950). Mannan and dextran yield different crystals at low and high temperatures, and there was not only a polymorphic difference, but a conformational difference in cellulose (Quenin and Chanzy, 1987). Curdlan appears to have three polymorphs--anhydrous, hydrated, and annealed. Hysteresis is the variable response of a system, e.g., a polysaccharide disperse system, to different processing and handling procedures, or the mode and order of application of a stimulus. The different procedures are referred to as pathways. Linear polysaccharides are prone to hysteresis traceable to different entanglements and disentanglements of the primary chain. A film made by hydrating a xerogel has a lower diffusivity than one made by dehydrating a hydrosol to an identical water content, because the former dispersion contains smaller pore diameters than the latter. The different hydration and dehydration pathways followed by the same dispersion earmark a p h e n o m e n o n called sorption hysteresis. Similarly, rehydrated mashed potato has an eating texture different from that of freshly mashed potato at the same moisture content, because of hysteresis. Viscosity hysteresis arising from different treatment histories of the same disperse system can sometimes be a quality-assurance and quality-control problem. A hysteresis loop is shown for methylcellulose in Fig. 3, where two viscosities are observed at one temperature in a critical temperature range. Mindful of the energy requirement for dispersion of a polysaccharide solute in water, syneresis is the slow, spontaneous separation of liquid from a gel, as the solid phase attempts to return to its energy ground state. This p h e n o m e n o n is a quality defect, because it foreshadows solute sedimentation.

III. P h e n o m e n o l o ~ / Phenomenology is the study of behavior without explanatory structures and mechanisms. The chemically similar polysaccharides lend themselves to phenomenological study, because of the commonality of many of their properties and responses to ambient stimuli: for example, irrespective of their chemistry, polysaccharides are generally dispersible in water and indispersible in acetone; their "qi is concentration dependent within a critical range, and, with few exceptions, they respond identically to heat.

20

I. Origin and Characteristics of Polysaccharides

Figure 3 Thermal gelation of methylcellulose in aqueous solution (100 mPa s at 20~ and heating rate 0.25~ min-a). Reprinted with the permission of the Dow Chemical Co., Midland, MI.

The p h e n o m e n o l o g y of amorphous cellulose and a m o r p h o u s starch (amylopectin) transcends the conventional classification based on chemistry. Although opposites in branching and bonding, they hydrate equally well at r o o m temperature and develop high xli, whereas crystalline cellulose and crystalline starch (amylose) do not. Cellulose and starch crystallites possess other properties in c o m m o n m m o r e with each other than with their generic a m o r p h o u s analogs, A carrageenan-water sol (polyanionic) is visually indistinguishable from a Konjac m a n n a n - w a t e r sol (nonionic): each has the appearance of a thick paste at refrigeration temperatures.

IV. Property and Function Modifications The DS, DE, and DP are intrinsic to the polysaccharide molecule, exercising influence on the solute's intra- and intermolecular interaction and interaction with water. Physical processes have extrinsic influences, insofar as they affect directly the DS, DE, and DP, and indirectly the dielectric property of

IV. Property and Function Modifications

21

water. Industrial suppliers offer a wide variety of DS, DE, and DP polysaccharides for diverse uses involving water solubility, dispersion stability, clarity, film strength, etc. A D S < 0.1 is enough to modify the properties of starch (Dautzenberg et al., 1994). The cold-water dispersibility, clarity, and stability of starch are best at DS = 0.02-0.2. The most widely used CMC has DS = 0.7 (Hercules, Inc., 1980). Some pectins do not gel with 65% soluble solids and acid outside a DS range of 43-85% (Pilgrim et al., 1991). There are many ways to influence the polysaccharide response intrinsically through the DS, DE, and DP, and extrinsically through cosolutes and the solvent.

A. Acid

The 1,4-c~- and 1,6-oL-glycosides are stable for moderate intervals in acidic media at the strengths (pH 3.3-4.2) and temperatures (0-121~ associated with processing low pH foods (e.g., fruit juices, jellies, sauces). With prolonged heating, reducing glycosides transmute and depolymerize in proportion to the severity of the process. The polysaccharide anionic character is strongly inverse to pH. The [3-glycosides, glycuronans, and pyranoses are more stable to acid than the oL-glycosides and furanoses (Whistler and Smart, 1953; Marsh, 1966); 1,3-glycosides are unstable. Amylopectin is more acid-stable than amylose, identical in all but branching and anomeric configuration. Pectin is the relatively acid-stable hydrolysate of protopectin, its natural, acid-labile, insoluble precursor. Completely demethylated galacturonans (pectic acid) are yet more stable than methylated galacturonans and starch. Starch is partially hydrolyzed under mildly acidic conditions to "thinboiling" fluids that are less viscous and more translucent than the untreated starch dispersion, because of the reduction in DP. Strongly acidic media (pH < 3.0) promote more extensive lowering of the DP: acid-stable polyanions may precipitate directly without passing through the transitional gel state. B. Alkali

Polysaccharides are differentially stable to colddilute alkali, which facilitates classification as alkali-stable and alkali-sensitive (Reeves and Blouin, 1957). High concentrations of alkali swell fibrils, shift conformation, and depolymerize the primary structure. The 1,4-[3 bonds are more alkali-resistant than the 1,4-or bonds. Nonreducing glycosides are more stable than reducing glycosides. Ester and sulfate groups are easily hydrolyzed. Polysaccharide [3-elimination is a mild alkaline depolymerizafion reaction, whereby a molecule of water is introduced at the site of rupture where a double bond forms. An esterified hydroxyl (on C-6 of the monomer ring) is

22

I. Origin and Characteristics of Polysaccharides

a prerequisite for [3-elimination (Albersheim et al., 1960). Metko and McFeeters (1993) studied the reaction and discovered that pectin with a higher degree of methylation degraded faster. Amylose and amylopectin are easily solubilized by 1-2% NaOH. In an oxygen-free atmosphere, heated amylose isomerizes to saccharinates, but degrades rapidly in air; amylopectin degrades to its 1,6-a branch points. Advanced stages of alkalization (with heat) result in rearrangement, decomposition, and resynthesis to yellow-brown chromophores (caramel); the degradation is accelerated by phosphate and acetate (BeMiller, 1965a). Glycogen, the so-called animal starch, chemically similar to amylopectin, is not affected by 30% boiling NaOH, a concentration that destroys most biological tissues. Cellulose is inert to all but the most drastic alkaline conditions. Mild alkali (0.5 N, 100~ lowers the DP in time by progressively lysing the reducing-end monomer. It and concentrated alkali react heterogeneously 2 in numerous integrals of temperature, concentration, and time to give swollen fibers called alkali-cellulose that ultimately yields cellobiose, cellotriose, and cellotetraose (the soluble dimer, trimer, and tetramer, respectively, of glucose). In the terminal stages, numerous short-chain acids (Richards, 1963) appear in the reaction mixture. Cellulose homogeneous reactions recently have been made possible in N,N-dimethylacetamide-LiC1 solvent (McCormick and Shen, 1982). Partial demethylation enhances the physical reactivity of many natural polysaccharide esters. NaOH (0.1 M) is the reagent of choice for partial demethylation, with minimum impact on the primary structure. Deacetylated guar gum interacts more strongly with xanthan gum than does acetylated guar gum (Lopes et al., 1992). Alkali causes galactomannans to become more coiledma transition attended with a loss of T]i not attributable to depolymerization (Hui and Neukom, 1964). Alkaline pretreatment of K-carrageenan enhanced the latter's gel-forming ability and raised the gel melting temperature (Watase and Nishinari, 1987). At pH 14, cellulose ionizes to a polyanion (cellulosate) and the hydronium counterion (H30+). Counterions are also called gegenions.

C. Oxidants

The controlled oxidation of otherwise stable polysaccharides increases their polarity. By oxidation in an alkaline medium, the C-6 hydroxyl group is converted to a carbonyl or carboxyl group, resulting in enhanced am2. In this context, heterogeneity refers to the occurrence of the chemical reaction in two p h a s e s - - l i q u i d (solvent) and solid (cellulose). The reaction is h o m o g e n e o u s if the two reacting phases are completely miscible.

IV. Property and Function Modifications

23

phiphilicity and affinity for cations. The concentration of aldehydes, ketones, and acids depends on the severity and duration of the oxidation. Oxidized starch loses its ability to gel, thereby making low xli dispersions. Glucose dialdehyde is a glucose oxidation compound used to crosslink agarose (linear) and dextran (branched) for their performance as adsorbents. Oxidized celluloses are a substitute for glues manufactured from animal by-products.

D. Enzymes The bond specificity of enzyme hydrolysis and the very mild reaction temperature not only ensure minimum degradation of polysaccharides, but safeguard against many of the secondary and side reactions associated with chemical hydrolysis. The choice of enzyme(s) has consequences for gel firmness and ion sensitivity. The initiation and progress of enzyme hydrolysis may require block or random sequences (Rexova-Benkova and Markovic, 1976; Ishii et al., 1979). Enzymes from different sources may not be catalytic to related structures: for example, pectin, alginic acid, and gum tragacanth are three galacturonans not demethylated by the same esterase. The cationbinding capacities of enzyme-deesterified pectin differ from those of chemically deesterified pectin (Yalpani, 1988). Enzyme activity may be enhanced by a chemical pretreatment, as in the example of alkaline hydrogen peroxide on corn fiber, where the hydrolytic rate was increased by a factor of 1.6 (Leathers, 1993). Some enzymes depolymerize polysaccharides by [3-elimination. Polysaccharides can be completely depolymerized by complementary enzyme action (Leathers, 1993): for example, starch is completely hydrolyzed sequentially to glucose, beginning with the amylases, or-Amylase excises amylose and linear side chains from amylopectin to yield a preponderance of maltotriose, maltotetrose, etc., and branched oligosaccharides called limit dextrins: the substrate qfli is significantly lowered. Without e~-amylase, long segments are not converted to short-chain dextrins, [3-Amylase (syn. diastase) reduces amylose and linear fragments to maltose. Neither enzyme can hydrolyze 1,6-a bonds, which require 1,6-ot-glucosidase in whose absence the depolymerization terminates in branched oligosaccharides. In plant material, cx-amylase is normally low and [3-amylase is normally high. Commercial preparations are intentionally prepared to contain all the enzymes necessary for complete starch hydrolysis. [3-Glycosidases hydrolyze the [3-glycans. In fermentations, the amylases are either present in or added to the polysaccharide substrate at activity levels sufficient to hydrolyze starch to maltose. Yeast does not generate the amylases, So malt (germinated grain) is relied on to augment their concentration. The importance of yeast is to produce maltase for converting maltose to glucose.

24

I. Origin and Characteristics of Polysaccharides

Glucoamylase depolymerizes starch, beginning at the nonreducing end, through the 1,4-a and 1,6-e~ linkages, terminating at (the theoretical) 100% glucose; thereby, liquid sweeteners with a high-dextrose equivalent 3 of 92-95 are obtained. Glucose isomerase isomerizes some of the glucose to fructose, yielding high-fructose (corn) syrups with yet greater sweetening power. Recent developments in carbohydrase technology include antistaling c~amylases, immobilized amylases, and amylases with increased tolerance of heat and acid (Hebeda and Teague, 1994). E. Chemical Substituents

Mention has already been made of the numerous effects attendant upon chemical substitutions on the polysaccharide linear chain. Natural branches impart a dispersion stability to amylopectin that is not afforded amylose. One only has to compare cellulose ethers, deesterified chitin, and the lysis product of protopectin with the underivatized parent compound to appreciate the impact of chemical substituents on functionality. The loosening of compact, parallel structures with alkyl, hydroxyalkyl, and alkoxyl groups facilitates hydration and transforms insoluble, refractory polysaccharides to soluble, reactive polysaccharides. Not only do these substituents obstruct the crystallization tendency, they almost always confer secondary functionalities like ~q enhancement and foam, suspension, and freeze-thaw stabilization. Whereas extracted, purified cellulose is comparatively physically inert, the carboxymethyl derivative is among the most functional of polysaccharides, and the hydroxyalkyl derivatives are among the most unusually alcohol tolerant. Propyleneglycol alginate does not gel because polypropylene glycol hinders the prerequisite ordering: this polyester is more acid stable than algin, a property attributed to bulky substituents (Wong, 1989). Polysaccharide functionality is a variant of the nature and distribution of the substituents. A polar substituent makes a polysaccharide less hydrophobic, i.e., more hydrophilic, and vice versa. A uniform distribution of - C H 2 C O O - on the cellulose primary structure results in smooth, nongrainy nonthixotropic sols. The probability of nonuniform substitution increases at a low level of substitution of cellulose-containing crystallites, because the reagent is unable to penetrate the crystallite, regions (DuPont). Deesterification of natural polysaccharides accomplishes the same result as chemical derivatization, to wit, the critical DE range necessary for highmethoxyl pectin gelation. Robinson et al. (1988) discovered that the normally gelling, branched polysaccharides studied did not gel when side chains were removed, because the unsubstituted main chain did not subsequently adopt an ordered conformation. Their explanation was that the polydispersed 3. Dextrose equivalent refers to sugar concentration reported as dextrose, i.e., 100 (mg reducing sugar per milligram of dry substance).

IV. Property and Function Modifications

25

linkages might still have prevented ordering. The acetyl group found naturally in many polysaccharides is gel-inhibiting at concentrations above approximately 2.6% (Pippen et al., 1950). Chitin, an acetylated aminocellulose, is far less reactive than chitosan, the deacetylated derivative. F. y-Radiation Polysaccharides are degraded by ionizing radiation, with crosslinking as a possible side reaction, via free radical mechanisms depending on the water content (Bellamy and Miller, 1963). Radiolysis occurs at all doses, sometimes with an initial increase in ~1, as more polysaccharide becomes solubilized (King and Gray, 1993). Low doses are an effective means of obstructing aggregation. ~/-Radiation has been shown to be a feasible way to convert higher DP alginates to lower DP alginates with improved properties (King, 1994). Loss of ~1 accompanying such depolymerizations can transform pseudoplastic fluids to Newtonian fluids. In carrageenans, radiolysis was shown to be initially rapid and then to decrease to a constant, low, radiation-insensitive DP: the rate was faster in solution than in the solid state (Marrs, 1988). G. Micromolecules The solvent quality of water deteriorates by additions of alcohol and salt, singly and combined. Polyanions are especially sensitive to cations and are invariably precipitated by the di- and polyvalent species in sufficiently high concentration. Charge suppression by nonsolvents and cations decreases the polyanionic coil volume (v c) and simultaneously reverses the solute-solvent interaction through 0 to precipitation. Micromolecules can alter the dispersion rheology (Pastor et al., 1994) of charged an neutral polysaccharides. 1. Salt

Electrolytes affect dispersed polysaccharides through water inactivation, specific ion binding, and polyanion neutralization. Each effect is valencedependent, but is less on neutral polysaccharides than on ionic polysaccharides. Di- and polyvalent cations gel or precipitate a constant amount of polysacchride at much lower concentrations than do monovalent cations. The precipitation reaction is used to advantage in isolating pectin with alkaline A13+, because this cation and polymeric forms of AI(OH) 3 readily precipitate and entrain pectinic acid from apple tissue homogenates. Other di- and polyvalent cation effects are crosslinking (Prud'homme et al., 1989) and an increased rate of [3 elimination over monovalent cations (Sajjaanan-

26

I. Origin and Characteristics of Polysaccharides

takul et al., 1993). Dilute concentrations of any of the valences can have a "salting in" or stabilizing effect on mostly dispersed polyanions; conversely, high concentrations can have a "salting out" or destabilizing effect. The polysaccharide sensitivity varies with the DS, DP, concentration, temperature, pH, and the order of additions of components to the solvent (Hercules, Inc., 1980). NaC1 at 0.6% doubled the viscosity of 1% CMC in water when added after instead of before dispersion of the CMC (Stelzer and Klug, 1980). NaC1 has virtually no effect on xanthan at any concentration in the practical use range (0.10-1.0%); in starch dispersions, this concentration level depresses gelatinization and raises the gelatinization temperature (Howling, 1980). The effect of NaC1 on starch gelatinization is complex (Lund, 1984). NaC1 lowers the phase-separation temperature of aqueous hydroxypropylcellulose (Hercules, Inc., 1971). Sodium K- and ~-carrageenan do not gel, but the potassium salts do. Potassium CMC and sodium CMC are water-soluble, but aluminum, ferrous, and ferric CMC are insoluble. Salt augmented the pH influence on the helix-random-coil transition in curdlan (Ogawa et al., 1972, 1973b); in gellan, it reduced helix aggregation at low temperatures and reduced coil dimensions at high temperatures (Miyoshi et al., 1994). Salt accelerated the rate of crystal formation in starch (Mita, 1992).

2. Sugar

The simplest explanation of the effect of sugar in sufficient concentration on polysaccharide dispersions is that it lowers the water activity and initiates hydrophobic interactions (Morris, 1985). In jelly-making with pectin, Michel et al. (1984) assign to 65% sugar the role of creating a poor solvent in which pectin molecules stiffen prior to aggregation, Stated differently, sugar competes with dispersed polysaccharides for water (Howling, 1980). Sugar also lowers the phase-separation temperature of hydroxypropylcellulose dispersions (Hercules, Inc., 1971), heightens certain intrinsic viscosity functions, stabilizes the ordered structure of gellan (Crescenzi and Dentini, 1988) and carrageenan gels (Nishinari and Watase, 1992), increases the rupture strength of carrageenan gels (Fiszman et al., 1986), raises the starch gelatinization temperature, and narrows the gelatinization range (Eliasson, 1992). The order of raising the gelatinization temperature is sucrose > glucose > fructose (Bean et al., 1978). Sugar is claimed to do more than simply lower water activity (Lund, 1984): it increases the number and size of junction zones in agarose gels (Nishinari et al., 1992). K-Carrageenan setting and melting temperatures were shifted upward with increasing concentrations of sugar (Nishinari and Watase, 1992). Edible plant tissues, notably of fruits and vegetables, may be saturated with sugar by a process called vacuum infusion, whereby tissues are immersed in the sugar solution and subjected to a vacuum emission of air from

VI. Summary

27

the intercellular space: the sugar solution replaces air. The occluded sugar retards the spontaneous deterioration that cellulose would otherwise undergo as a result of the dehydration and packing of microfibrils. Calcium a n d / o r pectinmethylesterase may be included in the sugar solution to enable simultaneous, cooperative binding of pectinic acid after demethylation of the carboxyl groups (Poovaiah and Moulton, 1982; Javeri et al., 1991). Retail packs of pectin for jelly-making utilize sugar as a diluent, so that the pectin does not cake and is suspended in water as discrete particles at the outset of mixing.

H. Homogenization Homogenization 4 is the uniform reduction in particle size by shearing, as a dispersion or suspension is forced through micron-sized apertures: simultaneously, viscosity and buoyancy are enhanced, thus prolonging the life of the otherwise unstable dispersion. Shearing can rupture chemical bonds and result in a high surface-to-charge ratio: this operation can convert a hydrated cellulose suspension to a hydrogel (Walter et al., 1977).

V. V o l u m e and the T h e t a Condition The volume relationship u n d e r 0 (V0) and non-O (V/) conditions is stated as V/=etV 0.

(1.2)

ot is called the expansion factor. In a good solvent, et > 1; it decreases in numerical value as the solvent becomes less good. The ideal condition for measuring intrinsic molecular properties is at 0 where ot = 1 and coil dimensions are unperturbed, i.e., i n d e p e n d e n t of the influence of solutesolute and solute-solvent interactions. The u n p e r t u r b e d state is difficult to reach by dispersed polysaccharides, given their spontaneous disposition to aggregate.

VI. S u m m a r y 0

Polysaccharides are mostly beneficial polymers, but a n u m b e r of them may be deleterious in some situations. Their properties and utilitarian value reside less in their chemical constitution than in their ability to change 4. A mechanical process unrelated to homogeneous solvent processes.

28

I. Origin and Characteristics of Polysaccharides

shape, size, properties, and function in numerous responses to ambient stimuli: water, cosolute, and heat exert and strongest influences. They are prone to self-association. The nature and magnitude of the responses, irrespective of chemistry, make them amenable to phenomenological study. The 1,4-a-polysaccharide random coil is a disordered, high-energy distribution of independently rotating segments, except when rotation is hindered by bulky substituents, side chains, and unfavorable surroundings. Given all the possibilities of rotation and interaction, it is logical to conclude that polysaccharide conformation is a dynamic property, constitutively based on the DP in a minor way, but largely dependent on and highly susceptible to ambient stimuli. Substituents and side chains modify many useful properties of refractory polysaccharides: deesterification of natural derivatives is sometimes beneficial.

CHAPTER 2

The Polysaccharide- Water Interface I. I n t r o d u c t i o n Water, occasionally modified by ethanol and salt, is the universal food solvent. V/ and v m are maximum in pure water and minimum in pure ethanol. Without hydration, polysaccharides cannot perform as plasticizers, thickeners, texturizers, stabilizers, crystallization inhibitors, and bulking and gelling agents. Crosslinking holds v m constant, irrespective of the solvent. All water relationships with solute theoretically disappear at 0. Three polysaccharide-water interfaces are shown in Fig. 1. The agarose interface is observed to be indifferent to water (Hayashi and Kanzaki, 1987), judging from its distinctly sharp, hydrophobic boundary. The methylcellulose and pectin boundaries are diffuse and clearly hydrophilic. The air bubbles on methylcellulose attest to its efficacy as a foam stabilizer. The interfaces depicted in Fig. 1 can be visualized on a macromolecular scale as the interfaces between dispersed polysaccharide molecules and water. There are at least four forces in effect at the polysaccharide-water interface, viz., van der Waals, ionic, hydrogen bonding, and hydrophobic (Ilmain e t a l . , 1991): van Oss (1991) adds a repulsive force due to Brownian motion, van der Waals forces are always attractive, because they depend on oscillating nuclei and electrons of paired molecules (Adamson, 1990); ionic and hydrophobic forces are attractive or repulsive, depending on the paired species; hydrogen bonding is attractive and paramount, because of the mutual affinity of hydroxyl and carboxyl groups. The strength of the hydrogen bond is 10-40 kJ mo1-1, intermediate between van der Waals (1 kJ mo1-1) and ionic (500 kJ mo1-1) bonds (Israelachvili, 1992). Urea is a hydrogen-bond breaker. The fundamental concept of a polymer molecule in solution is that it consists of a number of segments, each approximating the size of a solvent

29

W

0

Figure I Interface between water and an agar (a), methylcellulose (b), and pectin (c) gel. The agar (a) was prepared by pouring 1.5% hot sol into a 1-in.diameter plastic die and allowing it to cool. Methylcellulose and pectin (b and c) were prepared similarly. The dies were then immersed in ethanol and placed in Petri dishes in preparation for photographing. The photographs were taken after 6 months (a), 24 h (b), and 4 h (c). Note the sharp boundary in (a), the adhering air bubbles in (b), and the diffuse interface in (c).

II. Properties of Water

31

molecule (Flory, 1953). The attractive forces at the segment-solvent interface are stronger by far in an aqueous polysaccharide dispersion than in a solution of a synthetic polymer and an organic solvent. The forces generated by contiguous segments are described as short-range forces; these are more likely repulsive than attractive and therefore tend to expand v m . Long-range forces are generated by interaction between distant segments, and are repulsive and attractive, but more likely to be attractive and therefore to lessen v m . Short- and long-range forces are collectively called the excluded volume effect, because they outline a discrete volume for each moleculemthe excluded volume (Vex)mfromwhich all other molecules are barred. The V e x effect is claimed to be the only important interaction between uncharged polymers (Dautzenberg e t a l . , 1994). By convention, the Vex effect is positive when the net force is repulsive and is negative when the net force is attractive.

II. P r o p e r t i e s

of Water

The properties of H 20 obtain from the tetrahedral geometry of the molecule. The hydrogen atoms form an angle of 104.5 ~ with the strongly electronegative oxygen atom, and the tightly bound electrons induce an asymmetric charge distribution with a center of negative charge at the oxygen site and a center of positive charge at each hydrogen: this electronic configuration confers 40% ionic character on the molecule (Belitz and Grosch, 1986). In the pure state, H 2 0 has four other H 2 0 molecules attached to it (coordination number of 4). A. The Dielectric Constant

H 2 0 calculates to a dielectric constant (D o) of 78.54 at 25~ the highest of the ordinary solvents. Ethanol has D = 24.3 at 25~ Given its high polarity, H20 easily engages in dipole-dipole and ion-dipole reactions. B. Ionization

At neutral pH and 25~ one water molecule in every 10 million becomes ionized through the bonding of one of its protons to another water molecule (Baianu, 1992a); West and Todd (1961) estimate one molecule in every 550 million. In practical terms, every 1 g mol (18 g) of pure H 2 0 renders ]0 -7 g mol of hydrogen ion (10 -7 g of H +) and 10 -7 g mol of hydroxyl ion (1.7 X 10 -6 g of O H - ) . In aqueous solution, H + is hydrated to H s O + (the

32

2. The Polysaccharide-Water Interface

hydronium ion). H 2 0 is 10 times more ionizable at 60~ than at 25~ 5 and 40 times more ionizable at 100~ (West and Todd, 1961). There is evidence that H + exists also as (H904)+ (Baianu, 1992a). In any of its cationic forms, H 2 0 engages in i o n - i o n and i o n - d i p o l e reactions.

C. Activity The concept of a chemical potential is germane to a discussion of water activity (aw), which is technologically defined as the ratio of the equilibrium water vapor pressure over a solution or dispersion (Po) and the water vapor pressure over pure water (p~ Also by definition, the chemical potential of a solvent (P-0) or a solute (P~i) is the rate of change in energy of either with a change only in the molal content of that c o m p o n e n t in solution. According to Raoult's law (Williams et al., 1978), P~o = p o + R T l n ( p o / p ~

(2.1)

~zo is the chemical potential of the solvent in the vapor phase, p~0 is the standard chemical potential (potential of pure solvent), and p o / p ~ is a vapor-pressure function proportional to the mole fraction of solvent. In a dilute polysaccharide dispersion behaving ideally it follows that P~o = P~~ + R T

ln[no/(n

o + ni)].

(2.2)

n o and n i are the n u m b e r of moles of solvent (0) and solute (i), respectively, and n o / ( n o + n i) is the mole fraction (no)- Correspondingly, ni is the mole

fraction of solute (i) and in a binary dispersion, no + ni = 1 always. In terms of the mole fraction of solvent, Eq. (2.2) may therefore be rewritten as p~o= P~~ + R T

ln(1-~i).

(2.3)

At pure-water equilibrium, ~ i = 0 and

~o = P~~

(2.4)

~Zo/p~~ = a w = 1.

(2.5)

Whereas for a solution of small molecules, ln(1 - n i ) is an increasingly large negative n u m b e r as n i increases, ~0 is progressively lower than p~~ o as micromolecular solute is added. For aqueous polysaccharide sols, n i is exceedingly small and 1 - n i remains effectively constant in a large volume of water. 5. Calculated from data tabulated in Handbook of Chemistry and Physics, 62nd ed., CRC Press, Boca Raton, FL, 1981-1982.

33

II. Properties of W a t e r TABLE I W a t e r Activity (a w) of a Selection of Hawaiian Starchy Foods at Two Different Temperatures ~

26.7~

aw 6.7~

Moisture (%)

1.00

0.86

74.6

Kulolo c

1.0

0.84

55.7

Rice cake a

0.98

0.84

48.0

Manapua e

0.97

0.88

35.9

Foodstuff Poi b

aWalter and Seeger, 1990. bSteamed and mashed taro. CBaked taro and coconut pudding. a Steamed rice flour confection. eSteamed pork-filled bun.

The stability of a moist food is in proportion to the magnitude of a lowering of a w from 1. With the exception of lipid auto-oxidation, it is only at a w = 0.2-0.4 that the chemical (e.g., flavor changes, reaction rates) and biological (e.g., microbial growth, enzyme catalysis) stability of moist foodstuffs is assured. Lipid autooxidation rates are maximized by a reduction in a w , because of the enzyme's heightened activity in proximity to the substrate. Hydrosols do not show this low level of a w ; dried fruits show a w = 0.72-0.80 (Belitz and Grosch, 1986). From these facts, it is inferred that polysaccharides do not impart biological stability to liquid foods by a lessening of a w ; refrigeration is necessary (Table I), T being the alternative variable in Eq. 2.3. If n o and n i are expressed as their partial molal volumes (T o and ~i), the total volume of dispersion (V/) is

ni/(no

= +

(2.6)

+

(2.7)

= V/,

(2.8)

=

ni/V

i

is the solute molarity

ni/(no+ni)=CmVi

9

(c m)

and (2.9)

At equilibrium, the solvent and solute chemical potentials are equal, i.e., ~l,o= ~ i ,

(2.10)

34

2. The Polysaccharide-Water Interface

Eq. (2.2), in terms Of c o m p o n e n t i, may then be written Ix i -

Ix ~ = R T

In

Cm~)i.

(2.11)

D. Specific Heat The quantity of heat necessary to raise the temperature of a substance 1~ is the heat capacity of that substance. The specific heat capacity (synonymous with specific heat) of the substance is the heat capacity divided by its mass (weight). The specific heat capacity of water is 1 cal g-a ~ among the highest of solvents. Because of this, aqueous systems heat and cool relatively slowly. An infinitesimally slow rate of heat exchange is a necessary condition for thermodynamic reversibility. If water is cooled infinitesimally slowly without disturbance, it supercools; i.e., it persists in the liquid state well below its normal freezing point. The sudden imposition or withdrawal of a stimulus at the temperature of supercooling causes the supercooled water to revert to ice. In contrast, polysaccharide sols are supercooled by rapidly lowering the temperature with stirring.

E. Contraction and Expansion At 4~ 1 g of water has m i n i m u m volume (1.003 cm 3) and maximum density (0.997 g cm-~). The coefficient of volume expansion above 4~ to the boiling point (100~ is 2.1 • 1 0 - 4 c m ~ ~ the smallest in solvents. At a specific volume (reciprocal density) of 1 cm 3 g-a and molar mass of 18 g, the volume fraction (+o) is quite large relative to that of a dispersed phase.

F. Surface Tension Due to cr, water droplets completely surrounded by air assume a spherical shape. Sphericity not only offers the lowest ratio of surface area to volume, it also indicates a strong, uniformly distributed, unbalanced tension, directed toward the center, which must be overcome in any surface expansion. The spherical geometry of water droplets thus complies with the law governing entropy, stating a spontaneous minimization of surface energy, and hence surface area, at equilibrium. If cr is the energy necessary to expand or contract the surface area by 1 cm 2, its energy unit is elaborated into ergs per centimeter squared or dynes per centimeter. The cr of ethanol is 22.3 dyn cm-1 and that of pure water is 71.97 dyn cm -1 at 25~ the latter is the highest or ordinary solvents, varying only slightly with temperature. Any ethanol additions to water (0) therefore lower cr0 . The cr of solids is measurably less than that of liquids.

III. Polysaccharide-Water Interactions

35

At 25~ sucrose raises cr0 from 72.50 dyn cm-1 at 10% concentration to 75.70 dyn cm -1 at 55% concentration; at 20~ sodium chloride does so, from 72.92 dyn cm -a at 0.58% concentration to 82.55 dyn cm -1 at 25.92% concentration. 6 Nonequilibrium ~ gradients can be the driving force behind water migration from phase to phase or surface to surface. Marangoni flow (Ross and Morrison, 1988) is such transport from a region of low ~ (weak cohesive forces) to one of high cr (strong cohesive force). Nonequilibrium cr gradients can result from temperature, concentration, thermal and compositional differences, and compressions and dilatations of adsorbed films at interfaces.

III. P o l y s a c c h a r i d e - W a t e r

Interactions

Hydrocolloidal water is an integral part of the dispersed phase and travels at the same velocity with it; this is considered tightly " b o u n d " water. Yakubu et al. (1990) identified three other forms of water in potato and corn starch, viz., weakly bound, surface trapped, and bulk water. All forms were not present in potato starch containing less than 35% moisture, but were present in corn starch. Water, far removed from the solute surface ( u n b o u n d or free water in the outer volume), travels at a different rate from hydrocolloidal water (Lechert et al., 1981). Most polysaccharides affect the mobility and structuring of water (Blanshard, 1970) beyond the immediate interface (Barfod, 1988) to a thickness of several molecular diameters (Rickayzen, 1989). The reciprocal effect on flowing polysaccharides of surface-water immobilization and structuring is a contribution to distortions from sphericity, and hence to flow birefringence. Where there is no solute-solute association, macromolecules may act simply as a viscosity e n h a n c e r of the continuous phase: Barnes et al. (1989) call this p h e n o m e n o n neutral interaction. T h r o u g h what is called hydrodynamic interaction (Dautzenberg et al., 1994), the streamlines of hydrocolloidal particles flowing past each other affect each other. Tightly bound water apparently does not contribute much to a w (Yakubu et al., 1990). Free water is removable from a sol by freezing, while simultaneously, soluble trace components concentrate in the hydrocolloidally bound, unfrozen water, often to saturation. In a binary dispersion, there is usually one of five interfaces to consider, where a polysaccharide, for example, may act as a protective colloid. These interfaces are liquid-solid (sol), liquid-liquid (emulsion), solid-solid (mixed xerogel), liquid-air (foam), and solid-air (powder). In any of these systems at 6. Handbook of Chemistry and Physics, 62nd ed., CRC Press, Boca Raton, FL, 1981-1982.

36

2. The Polysaccharide-Water Interface

equilibrium, (to, i is the difference between the higher and the lower component (r and, consequently, at a water-polysaccharide interface, (r0,~ is always positive (Jirgensons and Straumanis, 1962): Cro,i -

(Yo -

(ri .

(2.12)

The function of a protective colloid is to lower (To, i to a minimum. In practical language, wetting is an attempt by a surfactant to accomplish this by lowering the contact angle, which enables liquids to spread over each other, on its mission to make the phases mutually miscible. Relative to solvent and component, the concentration of a protective colloid is quite low, but it accumulates at the interface, theoretically as a thin film. Micromolecules that wet surfaces dissolve completely in the solvent. One unique property of micellar surfactant electrolytes is their ability to solubilize some otherwise insoluble organic molecules (Adamson, 1990). With exceptions, amphiphiles lower or0,i and inorganic electrolytes raise it (Jirgensons and Straumanis, 1962: Hiemenz, 1986). Long-chain fatty acids and alcohols lower (to,i by decades as a function of their concentration. Traube's rule (Jirgensons and Straumanis, 1962) states that fatty acids lower (r0 in proportion to the chain length. Large concentrations of sucrose raise (to, i only slightly (Vold and Vold, 1983). Of the polysaccharides with varying degrees of surfactancy, methylcellulose is claimed to be the most efficacious; xanthan gum is more so as a random coil than as a helix (Young and Torres, 1989). The glycomannans are particularly adept at the prerequisite organization for surfactancy into lamellar liquid crystals at an oil-water interface (Reichman and Garti, 1991).

IV. Influences on Polysaccharide-Water Interactions Four features of a polymer solute figure prominently in the polysaccharidewater interactions, viz., bonding, branching, ionization, and nonuniformity of the repeating structure (Glass, 1986).

A. Bonding With mixed results, correlations have been attempted between polysaccharide composition and function. Beginning with simple sugars, sucrose (an ot-D-glucopyranosyl-[3-D-fructofuranoside) and maltose (an c~-D-glucopyranosyl-oL-D-glucopyranose) are truly soluble; cellobiose (a 1,4-[3-D-glucopyrano-

IV. Influences on Polysaccharide-Water Interactions

37

syl-[3-D-glucopyranose) and the trimer raffinose (1,4-a-D-galactopyranosyl1,6-O~-D-glucopyranosyl-l,2-[3-D-fructofuranoside) are not. The 1,4-a- and 1,6-Or-D-glycosides contain the most hydrophilic linkages, but the 1,4-[3-glycoside correlates with insolubility and crystallinity. Inulin, a biopolymer of 1,2-[3-D-fructofuranose, is soluble. The 1,3 bonding provides less symmetry than 1,4 and 1,6 bonding, and consequently less chain-chain associations, which ultimately enables better dispersibility. 3,6-Anhydro-ct-L-galactose is structural to the gelling of carrageenans.

B. Branching Amylopectin and glycogen, differing only in the frequency and length of branching at the sixth carbon of the glucose monomer, are readily hydratable, but amylose, the linear counterpart, is not. Guar gum, a gelling polysaccharide with many uniformly spaced et-D-galactopyranosyl monomers in the smooth region of the fine structure, has a higher water affinity than does nongelling locust bean gum with fewer unevenly spaced et-Dgalactopyranosyl monomers in the hairy regions. Scleroglucan, a 1,3-[3-glucan substituted with 1,6-[3-glucose, does not normally gel; yet curdlan, with a similar primary structure but without most of the glucose substituents, does gel. In the view of Yalpini (1988), the fact that the least branched galactomannans develop gels upon standing suggests that artificial branching does not improve hydration.

C. Ionizing Groups Ionization offers dispersed polyanions short-term protection from deposition through shielding with H 3 0 + counterions. The hydration of xanthan is enhanced by its containing a charged, trisaccharide side-chain repeating unit (Sanofi, 1988).

D. Heterogeneity Glass (1986) conceived of irregularities in a polysaccharide chain as possibly promoting dissolution. He cited the carrageenans as one example, admitting, however, that experience to date is inadequate to predict polysaccharide behavior based on chain heterogeneity. It is noteworthy that guar and locust bean gums (both heteropolysaccharides) are compatible with the widest spectrum of other polysaccharides.

38

2. The Polysaccharide-Water Interface

V. Polysaccharides as Adsorbents Micromolecules and ions, initially dissolved in the outer, flee-draining volume of a solution interfacing a solid surface including a polysaccharide surface, accumulate by diffusion (osmotic migration) across an imaginary, semipermeable membrane into the inner, adsorbed layer of water on the surface. For most solid-liquid systems, the accumulation theoretically ceases at a monolayer or equilibrium concentration (plateau value). At equilibrium, ~i is equal on both sides of the membrane. This conceptual behavior, called positive adsorption, is reminiscent of a concentration cell in which the diffusion rate of the migrating solute is proportional to the concentration (ci). Negative adsorption (desorpfion) is the reverse process. The Gibbs adsorption [Eq. (2.13)] for dilute solutions formalizes positive and negative adsorption of nonelectrolytes: Oc i = - ( R T ) -1 ci'OCro/OC i .

(2.13)

OCi is the excess concentration on the solid adsorbent surface per unit cross section, and OCro/OC i is the rate of change of cr0 with ci . The greater the surfactancy in the bulk solution, the lower is 0or0 and the higher is Oci. A s a consequence of positive adsorption, the layer of water on the solid surface passes from high (initial state) to low (final state) or, OCro/OC i becomes negative, and 8ci becomes positive. As Eq. (2.13) shows, the accumulation is inversely temperature-dependent, and the rate of change of surface tension with concentration is directly temperature-dependent. The surface of a powdered polysaccharide equilibrated in air is hydrophobic and resistant to wetting--a condition that poses difficulty in dispersal when cereal flour, for example, is mixed with water in the preparation of doughs and batters. Dispersion usually requires a large expenditure of mechanical energy.

VI. Polysaccharides as Adsorbates Equilibrium or monolayer adsorption of a polysaccharide as adsorbate is unlikely, except in the latter process, as a result of chemisorption, whereby valence forces extend to no more than one molecular distance. Instead, the first layer of polysaccharide provides an adsorption site for the second layer, ad infinitum, in a nonequilibrium process, until phase inversion. Macromolecules including polysaccharides do not desorb: they accumulate in multilayers with an increased rate of adsorption at higher temperatures.

VI. Polysaccharides as Adsorbates

39

The Freundlich equation, empirical in origin, relates positive adsorption to a power function of ci , as follows:

w/g=k'c~/k,

(2.14)

l o g ( w / g ) = 1 / k ( l o g ci) + log k'.

(2.15)

w / g is the weight of adsorbate per gram of adsorbent. The range of k is 0.1-0.5 (Daniels et al., 1970). The validity of Eq. (2.15) is proven when l o g ( w / g ) vs log c/ is a straight line at a constant temperature. When k = 1, the rate of surface accumulation is equal to the rate of change in solution concentration, and at equilibrium, there is a 1:1 distribution of ci between the surface and the solution. When k > 1, the adsorbent is highly efficient at accumulating ci; the opposite holds for k < 1. Equation (2.15) is most adequate to quantify adsorption of small electrolytes from solution over a considerable range of concentrations (Glasstone and Lewis, 1960). An important characteristic of Eq. (2.15) is the limiting value of w / g as k increases. The B r u n a u e r - E m m e t t - T e l l e r equation governing multilayer adsorption shows inflections above monolayer saturation as the adsorbate accumulates on a surface over an extended interval. Heller (1966) factored in the time variable: t/(w/g)

= k + k't.

(2.16)

w / g is now the weight of adsorbate per gram of adsorbent at any time t, k' is the slope, and k is the intercept. The inflections in polymer adsorption isotherms are explained by Adamson (1990) as possibly deriving from physical adsorption over a chemisorbed layer, which results in the observed isotherm being the sum of two isotherms. Another explanation is that different surfaces that have different adsorbabilities exist on the adsorbent. As adsorbate, the polysaccharide Oci is sensitive to the DP and polydispersity (Cohen Stuart et al., 1982). Higher DP polysaccharides are less kinetically active, are therefore slower to accumulate than lower DP polysaccharides because of the time taken for surface orientation, and are thus more inclined to stay adsorbed longer and reach higher concentrations. Agitation increases the rate of physical adsorption. From the foregoing discussion on polysaccharide dispersibility, it is safe to conclude that multilayer adsorption is antecedent to polysaccharide phase inversion and in some instances to sol-gel transition. Chemisorption is irreversible adsorption, which suggests valence bonding at specific sites on a surface. Transition metal ions, protein below its isoelectric point (positively charged), and di- and polyvalent cations are prone to chemisorption. As an adsorbate, a polysaccharide is modeled as orienting itself linearly in flattened conformations (Dickinson and Euston, 1991a), any of which

40

2. The Polysaccharide-Water Interface

may engage it in stabilization and destabilization. If only a segment is adsorbed, the remainder protrudes into the bulk liquid as tails, loops, and trains (Van de Ven, 1989): these localized conformations may be restricted to the interfacial layer or may migrate from it (Lips et al., 1991). Galactomannans, particularly, become adsorbed and organized on an oil-drop surface as lamellar liquid crystals that perform as steric and mechanical barriers to coalescence (Reichman and Garti, 1991). The surfactancy of xanthan was found to be related to the amount adsorbed (Young and Torres, 1989).

VII. Summary Polysaccharides interfaced with water act as adsorbents on which surface accumulations of solute lower the interfacial tension. The polysaccharidewater interface is a dynamic site of competing forces. Water retains heat longer than most other solvents. The rate of accumulation of micromolecules and microions on the solid surface is directly proportional to their solution concentration and inversely proportional to temperature. As adsorbates, micromolecules and microions ordinarily adsorb to an equilibrium concentration in a monolayer (positive adsorption) process; they desorb into the outer volume in a negative adsorption process. The adsorption-desorption response to temperature of macromolecules--including polysaccharides m i s opposite that of micromolecules and microions. As adsorbate, polysaccharides undergo a nonequilibrium, multilayer accumulation of like macromolecules.

CHAPTER 3

State- and Path-Dependent Properties I. Introduction A system approaching thermodynamic equilibrium in infinitesimally slow steps is characterized by reversibility of the steps without a change in the energy status of the infinitesimal transitions. At equilibrium, the integral of the variables is constant, so that a change in one limits change in the others. The integral is a state function, and the associated properties are state properties. If an integral of identical variables depends on the pathway, the properties are path-dependent and inexact. Sorption hysteresis, supercooling, and diffusivity, for example, are inexact properties. The thermodynamics of small molecules is predicated on equilibrium states arrived at in relatively short time intervals, making them essentially time-independent. Polysaccharide processes seldom reach thermodynamic equilibrium in a practicable time interval.

II. M a s s - V o l u m e - P r e s s u r e - T e m p e r a t u r e

Relationships

One mole of gas (n) has is volume (V) specified by temperature (T) and pressure (~r) according to the Charles-Boyle law, and any change in either of these two variables results in a corresponding change in the other, so that the following equation is satisfied: n = 'rrV / R T .

(3.1)

This equation is a mathematical statement that T / V is constant in any constant-pressure process and T/'rr is constant in any constant-volume process. Macromolecules in very dilute aqueous concentration imitate gas

41

42

3. State- and Path-Dependent Properties

molecules, in conformance with Eq. (3.1). In binary polysaccharide dispersions, conformational transitions of the dispersed phase introduce negligible volume change (AV) in the dispersion; however, the solvent phase (water) experiences noticeable AV at vaporization at 100~ and freezing expansion at 0~

III. Electrostatics and Electrokinetics A pair of polysaccharide molecules approaching each other in water exerts an interaction potential (~') that is the algebraic sum of the competing attractive and repulsive forces. ~', integrated over all pairs of molecules, is ~. This principle is embodied in the Derjaguin-Verwey-Landau-Overbeek (DLVO) theory of colloidal stability (Ross and Morrison, 1988). The equilibrium distance between the molecules is related to c i , the volume of the hydrated particles, ionic strength, cosolute, nonsolvent additions, temperature, and shearing.

A. Nonionic Polysaccharides Neutral molecules, dissolved, dispersed or suspended in a liquid medium, are in continuous random motion (Brownian motion) with a mean free path (x) and collision diameter (Xe), depending on c i and Vex effects. At a far separation distance, ~ is negative, increasing to 0 at x e , where repulsion counterbalances attraction and the amphiphiles are at dynamic equilibrium in a primary minimum energy state. At x < x e , the molecules repel each other and ~ is positive. High concentrations shorten x and make the collision rate nonlinear with ci (Hammett, 1952). A separation distance of x < x e is sterically forbidden without fusion. Coulomb's law of electrostatic attraction between two unlike charges (QI and Q2) states F = QIQz/Do x 2.

(3.2)

F is the force of attraction (Coulombic attraction), seen to be inversely proportional to the second power of x. Given the closer proximity of QI and Q2, small-diameter particles experience a much greater force of attraction than large-diameter particles. Between a point charge and a dipole, F varies with x -~ and between an ion and a dipole F varies with x-2 (Adamson 1990). Coulombic attraction between ions of opposite charge is of much longer range than van der Waals attraction between neutral molecules (Alberty and Silbey, 1992). The latter acts through 6-7-nm distances, decaying exponentially with the sixth or seventh power (Van de Ven, 1989) of x.

43

III. Electrostatics and Electrokinetics 1QQ2 .< . . . . . . . . . . . . . . . . . .

l-x~l

Q2 [

)~

x'

AGmix.=0

+AGmix. =o~

Stability

Metastability

Instability

Figure I Diagram of the correspondence between the distance (x) and stability of a polysaccharide dispersion containing solute particles (Q) in water. As the particles approach each other (Q2 approaching Qa), the interparticle distance x' is shortened and the dispersion passes through stages of metastability to x e , the collision diameter. For a freshly prepared sol, the particles experience the highest Brownian activity and the increase in energy is m a x i m u m ( + AGmix = 00), whence it declines to the primary energy state at equilibrium, when AGmix = 0. Aging is the process of + AGmix = 00 declining to AGmix = 0.

This inverse power integer is called the Born exponent (Gould, 1962). Simple power relationships (Israelachvili, 1992) apply to neither polysaccharide intermolecular nor surface forces due to the multiple effects of substituents, branches, kinks, etc. The total energy content (E) of an aqueous polysaccharide dispersion is stated in the general equation

E=

dx=QaQz/D o

dx/x 2= -Q,Qz/Do(1/x

+k).

The negative sign indicates a loss of energy up to x e as Q1 approaches Q2. Absolute energy states are difficult to measure, but more importantly, differential (d) and integral (A) changes are quantifiable, once a reproducible reference state has been decided on. Numerical changes are computed by subtracting initial assigned values from final assessed values. Although the law of entropy stipulates that molecules spontaneously strive to descend from a metastable E (at any x') to a primary m i n i m u m (at Xe), it is the goal of the food researcher to cause the kinetically active molecules to exist almost indefinitely in a metastable, kinetically active state at any x' > x e (Fig. 1). B. Ionic

Polysaccharides

It used to be thought that cations simply precipitated polyanions, but it was recognized later that electrolytes had special valence and solvent-mediated effects on a hydrosol other than neutralization of opposite charges (Holmes, 1922). It is now firmly established that ionization of the carboxyl and sulfuric acid groups in ionic polysaccharides, or adsorption of ions on neutral macromolecules, is an initial step in electrokinetic mechanisms of stabilization and destabilization.

44

3. State- and Path-Dependent Properties 1. D i s s o c i a t i o n

In the n o r m a l p H range of living tissues, weak carboxyl and sulfuric acid groups c o n n e c t e d to biopolymers partially dissociate in water and maintain equilibrium with an equivalent weight of H 3 0 + whose c o n c e n t r a t i o n is highest at the surface of the polyanions and decreases outward. The ionizing g r o u p may be integral to the primary structure as in pectin or p e n d a n t to it as in CMC. The degree of dissociation is directly related to the p H of the dispersion medium. In excess H 3 0 + or other counterions, ionization is depressed and the m a c r o a n i o n then behaves like its neutral c o u n t e r p a r t (Pals a n d Hermans, 1952) or as does a s a l t of the polyanionic acid. The acid strength is indicated by the dissociation constant ( K z) (Table I): the larger the K z , the stronger the acid. Interestingly, hyaluronic acid (not listed; p K = 3.23; Cleland et al., 1982), containing equimolar quantifies of [3-Dglucuronic acid and N-acetyl glucosamine, is a m o n g the strongest of the biological organic acids. Hyaluronic is an i m p o r t a n t c o m p o u n d in animal physiology, but is not intentionally a d d e d to food.

TABLE I The pH and Ionization Constant (K x) of Some Washed, Aqueous Dispersed Ionic Polysaccharides (_< I%) a

Polysaccharide

pH

Kz

CMC (0.50%) CMC (0.75%) CMC (1.0%) Gellan Gum arabic LM pectinb LM pectin b Algin HM pectin c HM pectin c AM pectin d AM pectind Xanthan Carrageenan Agar

7.0 7.0 7.0 7.2 5.1 3.2 3.1 6.2 3.2 3.6 4.4 4.3 7.0 7.0 6.7

10 -4.5 10 -4.5 10 -4.5

10- 4 . 4 10-5.9 10- 4.2 10- 4.o 10- 4 " 2 10- 3.9 10- 4.3 10- 4.9 10- 5.1 10- 4 " 2 10-4.9 10 -4"~

aWalter and Jacon, 1994. bDifferent grades of low-methoxylpectin (Hercules, Inc., Wilmington, DE). CDifferent grades of high-methoxyl pectin (Hercules, Inc.). dDifferent grades of amidated pectin (Hercules, Inc.).

III. Electrostatics and Electrokinetics

45

The identicalness of the ionization sites in a linear polyelectrolyte (Tanford, 1961) stimulated the interest of Walter and Jacon (1994) in a possible relationship between K z and M of ionic polysaccharides displaying the characteristic titration curve of a weak, monobasic acid. Without any theoretical assumption, Eq. (3.4) was derived from simple algebra by combining elementary principles of the dissociation theory of weak acids with polymer segment theory: ~=

[10-Pn* ]/{[10-2pn + 10-~pH* +pH)]f,.f,,}.

(3.4)

m

A segment factor f ' related M to the constant ratio between the carboxyl group and the monomer. With specific reference to pectin, a second factor (f") was later postulated to account for the degree of carboxylation. 7 Subsequently, a series of pectins determined to have an anhydrogalacturonan content of 68-96% ( f " = 0.68-0.96) did not always change the exponent of M. A CMC sample with a 0.75 degree of carboxylation, making f " = 0.25, increased the exponent by 1. Offering only theoretical surmises, Cesftro and Villegas (1996) refuted the M - p H relationship, without first addressing important aspects of it. Charge density and distribution, and location (pendant on ring) of the ionizing group, are probable factors to be considered.

2. The Electric Double Layer The solution arrangement of ionic polysaccharides as a primary layer of polyanions (the Stern layer) and a secondary layer of H s O + counterions creates a model of an electric double layer (also called the Helmholz double layer). The number of and average distance between adjacent charges on the polysaccharide surface are defined as the charge density. An imaginary shear plane separates the inner volume containing the fixed primary layer and the outer volume containing diffusely distributed H 3 0 + (and other cations present). This shear plane is estimated to be within a few molecular diame' ters of the particle surface (Hiemenz, 1986). Along this shear plane, a frictional coefficient (fc) is generated that is a property of the size, number, and distribution of segments. A potential (the zeta potential, ~)8 develops between the inner and outer volumes where, in the latter, ~ -- 0. The higher the charge density in the primary layer, the higher is ~, the more stable is the d o u b l e layer, and, consequently, the more stable is the dispersion. Ions migrating with water across the shear plane in the direction of the polyanion lower ~ and initiate destabilizing events. 7. Three carboxylation sites on a glucose monomer. 8. The reader is referred to Vold and Vold (1983) for the difference between the zeta and Stern potentials.

46

3. State- and Path-Dependent Properties

The swelling of plant parenchyma tissue has been ascribed to an electric double layer, which depends significantly on the type of counterions that constitute it: monovalent ions cause more swelling than do divalent ions (Shomer et al., 1991), probably because of their larger ionic radius. The electric double layer has been compared to a capacitor (Jirgensons and Straumanis, 1962)--a single device for storing positive and negative charges separately. An electrical potential is created by the charge separation across a narrow space where an inserted dielectric becomes polarized, effectively augmenting Q / ~ (defined as capacitance C), although the quantity of + Q and - Q is not changed. The augmentation results from cancellation by the dielectric's positive end of an equivalent fraction of - Q and by the negative end, of an equivalent fraction of + Q, thus effectively lowering and raising C. The dielectric constant of a substance is defined as the ratio of Q / ~ , measured with the substance inserted in the space between the charged leaves of the capacitor and Q / ~ measured in vacuo. In an ionic polysaccharide dispersion, water is the dielectric and C is the double-layer stabilizer. Electrolytes affect ~ and the charge distribution. The fundamental equations (Jirgensons and Straumanis, 1962) are =

/Do ,

x =k'(Do/is)

~

(3.6)

i s is the ionic strength and x here is the double-layer thickness or Debye length, an important property of the double layer (Cabane et al., 1989). For some hydrocolloids destabilization occurs below a critical g. In the DLVO theory, the Debye length is the distance over which ~ falls to 1/e or 37% of its value at the surface. The "salting in" and "salting out" of polyelectrolytes are related to i s and x. At a critical i s, x = 0. Equations (3.5) and (3.6) are in harmony with the common observation that water (high D o) normally stabilizes and ethanol (low D) destabilizes an ionic polysaccharide dispersion. Alternatively stated, ( C ) w a t e r > ( C ) e t h a n o 1 . Equation (3.6) confirms what is already known from experience: Ca 2+ and A13+ have greater flocculating power than Na I + and K 1+. In the other direction, polysaccharide polyanions are less inclined to disperse in water containing multivalent ions than in water containing monovalent ions. 3. Electrophoresis, Electroosmosis, and Streaming Potential

Given the charge, polysaccharide polyanions are electrically conducting in sols and gels, and move in an electrical field (electrophoresis) in compliance with the equation (3.7)

IV. Thermodynamics

47

rio is the solvent viscosity, f~ is the electrophoretic mobility, small for macroions relative to microions, and ~ is the applied electromotive force across x. Neutral polysaccharides do not migrate in an electrical field, except as a moiety of an ionic complex or when they adsorb ions. Electrophoresis is a useful m e t h o d for studying heterogeneity (Aspinall and Cottrell, 1970). In electroosmosis, solvent travels with the charged species and the volume (v) migrating per unit time in a standard cell is v=

Dor

(3.8)

o.

A is the conductivity of the solution. A streaming potential ~ is established by a confined solution flowing under pressure through small-diameter pores and capillaries. It is believed that the confining walls, typically glass, become charged with O H - , thereby initiating the potential. 4. T h e D o n n a n D i s t r i b u t i o n

Diffusion across a semipermeable m e m b r a n e causes an increment of Na + (y'), for example, to migrate from the outer solution containing Na +C1- at concentration y toward the inner solution containing the polysaccharide polyelectrolyte salt (Na+P - ) initially at c i . The migrating Na + is accompanied by an equal concentration of C1- to maintain electrical neutrality. At equilibrium, Na + and C1- on the P - side are ci + y' and y', respectively, and the quantity of each in the outer volume is y - y ' :

y'(c,+y')

=

y'=yZ/(ci+2y

(3.o) ).

(3.10)

As shown in Eqs. (3.9) and (3.10), the concentration of migrating cations is a squared function of the adsorbate concentration in the outer volume of the solution. The Donnan distribution is a source of serious error when determining M of ionic polysaccharides by m e m b r a n e osmometry. Polyanions may be freed of cations and excess H 30 + by electrodialysis.

IV. T h e r m o d y n a m i c s Classical thermodynamics discourses equilibria in solutions of noninteracting micromolecules, within and between phases, under constant and variable concentration, volume, temperature, and pressure. Deviations observed with macromolecules necessitated new theory modeled after a lattice in which segments of linear macromolecules, instead of the whole molecule, interchange positions with identical segments and with solvent molecules without

48

3. State- and Path-Dependent Properties

bond rupture. Each segment, first estimated at about 25 chain atoms long (Powell and Eyring, 1942), is configured by its immediate neighbors to which it is bonded. The minimum length that a freely rotating polysaccharide segment can be is the monomer.

A. Enthalpy The first law of thermodynamics (enthalpy) expresses the equivalence and interchangeability of the different forms of energy (heat, work, etc.), so that a molar mass of polysaccharide, for example, undergoing transformation from A to B, absorbs or evolves an increment of energy (AE) expressed (Glasstone and Lewis, 1960; Knight, 1970) as (3.11)

A H = AE + A(pV) - TAS.

AH 9 is the total energy exchange, AE is the internal energy change, and AS is the change in entropy (vide infra). A(pV), having dimensions of work (force times distance), 1~ is the energy expended in the transition, e.g., nonrecoverable pV work during viscous flow. At constant pressure, p AV is solely the work of expansion. Wunderlich (1990) adds an extra term to Eq. (3.11) to account for p OV necessary to promote a conformadonal change of random-coil polymers. A negative AH indicates an exothermic process and a positive AH indicates an endothermic process. Under isothermal conditions, no heat is gained by the dispersion or lost to the surroundings, all exchangeable energy is AH, and OV is accompanied by a corresponding change in AH [0(AH)], i.e., O(AH) =pOV.

(3.12)

For polysaccharide dispersions, OV is exceedingly small relative to V/. Equations (3.11) and (3.12) are mathematical propositions that the exchangeable energy stored in a dispersed polysaccharide solute is equal to the energy absorbed from an external source and any increase in surface area of the solute is consequently a repository of + AE. Conversely, aggregation and desorption correspond to a loss of energy, felt as heat in the latter occurrence ( - A E ) when a dry polyaccharide powder is wetted (positive adsorption). In a nonisobaric coil-stretch transition, (B.1B)

A H = niRT ln( pA/PB ) , 9. A H = ( H o - H i ) A - ( H o - H i )

B.

10. p V equals force per centimeter squared times centimeters cubed, which cancels to force times centimeters.

49

IV. Thermodynamics

where PA and P8 are the respective equilibrium pressures at states A and B. At constant V~ and p, A H is at once equated with its mechanical equivalent, the heat capacity (Cp,v):

AH=Ct~,vAT.

(3.14)

AT is measured between an initial temperature (TA) and a final temperature (T~). Experimentation has shown that A H of polymer solutions is not very large, depending only weakly on T (Allcock and Lample, 1981). One mole of water has A H = 0.99828 cal at 25~ and 1 atm. The Clausius-Clapyron equation [Eq. (3.15)] is of incidental interest, because it states the molar relationship of water in a dispersion in equilibrium with its v a p o r - - t h e definition of a w [Eqs. (2.1)-(2.5)]:

log(pB/Pa ) -- +__(AHvap/2.303R)( AT)

-1 .

(s.15)

AHva p is the heat of vaporization of water.

B. Entropy A descriptive definition of entropy (S) is that it is the a m o u n t of energy in a system unavailable for exchange (Glasstone and Lewis, 1960; Knight, 1970). A change in entropy (AS) from state to state is the practical thermodynamic variable for indexing the extent of randomness. AS is a function of the n u m b e r of polymer segments per unit volume of dispersion and of the distance from a surface or from each other (Van de Ven, 1989). The most kinetically active macromolecules are the most randomly dispersed and possess the highest AS. R a n d o m coils are high AS conformations (Poland and Scheraga, 1970) vis-h-vis rods a n d helices. - AS is typical of sol-to-gel and amorphous-to-crystalline transitions. In an isothermal, reversible physical event such as solution of a micromolecule in water, A H = T AS.

(3.16)

AS is related to Cp, v as follows:

AS = niCp, v l n ( T s / T A ) .

(3.17)

C. Free Energy of Mixing For the spontaneous merger of two phases to occur, the following condition must prevail: Gmi x ~_ (Gsolven t + Gsolute),

(3.18)

3. State- and Path-Dependent Properties

50

where ami x ~___Gsolvent -3t- asolute are, respectively, the Gibbs free energy of a solution, solvent, and solute. Spontaneous dissolution requires that AGmix be negative. The dissolution of polymers including polysaccharides must almost always overcome a condition of Gmix > (Gsolven t --I--asolute). In terms of AH and AS, mGmi x ~-- m E + A ( p V )

(3.19)

- T mSmix,

(3.20)

mGmi x = m n m i x - Z mSmix ,

where AHmi x equals Hsolven t minus Hsolute and ASmix equals Ssolven t minus Ssolute o The unit of AH, AS, and AGmix is Joules per Kelvin. 11 The 0 temperature is that temperature where AHmix = 0 and polymer dissolution exhibiting ideal behavior is instigated by AS only. By itself, ASmi x is incapable of predicting spontaneity and randomness; this is demonstrated in crystallization and helix formation that anomalously result in a high degree of order ( - A S ) , but are nevertheless spontaneous processes more significantly driven by a loss of latent heat ( - A H ) . For some constant-temperature, physical processes, e.g., phase separation and sedimentation, there is a corresponding 0(AGmix) for every op: 0(AGmix) = VOp.

(3.21)

Contemporary polymer theory considers segments of the primary structure to be the statistical unit comparable in size to that of solvent molecules. The large number of segments in polymers and the small scale of AGmix, AHmix, and ASmix allow their thermodynamics to be preferably described statistically (Smith, 1982), thereby permitting the following equations: AGmi x -- R T (

X o T t o + i -.[- Tt o I n

~)o +

ni In

+i),

(3.22)

AHmi x = R T X o n o + i ,

(3.23)

ASmix = - R ( n o In +o + ni In ~)i).

(3.24)

X o is a positive, inverse temperature-dependent interaction parameter per

solvent molecule (Allcock and Lampe, 1981). The Boltzmann law computes to a configurational AS governed by Eq. (3.22). A configurational AS represents dissolution of a perfectly ordered, pure solid polymer in pure solvent (Allcock and Lampe, 1981). van Oss (1991) cautions against designating physical processes as AH- or AS-driven unless careful microcalorimetric measurements have been made, because many thermodynamic suppositions (imputed to modeling or intuition) have not been substantiated by experimentation. Although descriptive analyses of 11. One joule equals 0.239 cal.

V. Kinetics

51

A H , AS, and AG may help to elucidate a mental picture of events, the thermodynamic status of most polysaccharide dispersions depends on the processes it u n d e r w e n t to achieve a desired end-product or use.

D. Irreversible Thermodynamics Nonequilibrium, time-dependent processes manifest mostly as transport and relaxation (Wunderlich, 1990) describe polysaccharide events. The terminal outcome of transport is a change in position (potential energy), and that of relaxation is restoration exactly or approximately to an initial energy state. Because polysaccharide events are nonequilibrium processes, the addition or subtraction of energy is necessary for reversibility.

V. Kinetics The inordinately long intervals required for many polysaccharide events to approach equilibrium necessitate that time and rate changes be more useful considerations. Kinetics is the study of the rate at which molecules arrive at or depart from an equilibrium state. The kinetics of large molecules is governed by the modification of rate laws applied to micromolecules.

A. Diffusion The unidirectional diffusion of solute is a function of cross-sectional area (A) and time along a concentration gradient (Oci/Ox):

= - A D ( O c i / O x ), D = (RT/N)(1/6pi

(3.25)

Xlor).

(3.26)

is the a m o u n t diffusing, D is the diffusion coefficient (diffusivity) in dimensions of distance -2 time-1, N is Avogadro's number, r is the particle radius, and pi = 3.142. The negative sign indicates decreasing concentration at the point of origin. Equation (3.25) is known as Fick's first law of diffusion. The flux of the solute is the rate of change of ~ with time across A. In reality, D is itself a function of ci and possibly Oci/Ox (Geddes, 1949).

52

3. State- and Path-Dependent Properties

Equation (3.26) is adapted to nonspherical particles by multiplying D by a dissymmetry factor (Geddes, 1949). D is related to the frictional coefficient fc of a macromolecule, given by (Williams et al., 1978) (3.27)

D =RT/Nfc

and to AH by (Hannay, 1967) D = D A exp( - A H / R T ) .

(3.28)

D is inversely related to the incidence of junction zones in hydrogels and directly related to the uniformity of the distribution of pores (Silberberg, 1989). Major difficulties in applying Eqs. (3.25)-(3.28) to polysaccharides arise from their polymolecularity and Vex effects, because molecules of different sizes diffuse at different rates and interactions preclude the fundamental i n d e p e n d e n t motion implicit in Fick's first law.

B. Order

of Reactions

A binary dispersion of polysaccharide and water is effectively unimolecular, given the constancy of c o and +o- Such a system is represented by c a =Co ekt,

(3.29)

2.303 log( ca/c o) = kt.

(3.30)

k is the rate constant. When c1

- - "

2c 0 , Eq. (3.30) reduces to

t0.5 = 0.693/k.

(3.31)

t0.5 is the length of time (half-life) required for 50% of any quantity of solute to accumulate ( + ) or be dispersed ( - ) , in a first-order process. At true equilibrium, AGmix =

niRT ]n(cl/co).

(3.32)

n i is the n u m b e r of moles of i distributed between cI and co .

Starch gelatinizationma unimolecular occurrencemfollows pseudofirst-order kinetics after an initial time lag (Okechukwu and Rao, 1996a).

VI. Hydrodynamics

53

VI. Hydrodynamics Similar to Eq. (1.2), the through or:

vc

of a polysaccharide random coil is related to

Vc = OLVo .

vo

(3.33)

Above O, ot > 1. As 0 is approached, solvent departs the interior of the coil (Hiemenz, 1986), the initially non-free-draining water becomes progressively indistinguishable from the free-draining water, all interactions cease, and ot = 1. 0 is hardly ever achieved in polysaccharide dispersions, considering the numerous opportunities for intra- and intermolecular bonding o f - O H and - C O O H groups that drive the spontaneous tendency to lessen the surface area by aggregation. A free-draining macromolecule is conceived as being in a more extended than coiled conformation, thereby permitting water to flow in streamline along the surface, unencumbered by adsorbed water. The superimposition of charge on the coil complicates the polyanionic response, which is then a function of the charge density and the uniformity of the distribution.

A. The Imaginary Shear Plane The solute-water interaction extends 1-3 nm (Israelachvili, 1992) and decays exponentially with distance (Van de Ven, 1989). Non-free-draining water is water within this distance traveling with the same velocity as the particle nucleus. At the interface between the non-free-draining (bound)water and the outer volume of free-draining water traveling at a different velocity, an f c [Eq. (3.27)] is generated. In this sense, hydration and the imaginary shear plane have enormous ramifications for human oral sensations elicited by dispersed polysaccharides.

B. The Equivalent Hydrodynamic Sphere The classical thermodynamic and kinetic model is that of a rigid sphere impenetrable by water. A spherical geometry has been observed in many polysaccharide systems, notably hyaluronic acid-protein complexes (Ogston and Stainer, 1951), dispersed gum arabic (Whistler, 1993), and spray-dried ungelatinized starch granules (Zhao and Whistler, 1994). Spherulites of short-chain amylose were obtained by precipitation with 30% water-ethanol (Ring et al., 1987), and spherulites of synthetic polymers were obtained

54

3. State- and Path-Dependent Properties

during the initial stages of crystallization (Khoury and Passaglia, 1976). To accommodate topologically linear macromolecules, the concept of an equivalent hydrodynamic sphere, responding identically to external stimuli as a linear molecule, was introduced (Tanford, 1961). The polysaccharide model is that of a random coil approximating the peripheral outline of a s p h e r e - - t h e equivalent hydrodynamic sphere. The radius of this sphere is the hydrodynamic radius (Rh).

VII. Free Volume Early theory propounded the existence of holes in a liquid that accommodated flow, as molecules " j u m p e d " from hole to hole (Eyring, 1936). Modern theory perceives spaces in a polymer melt originating from randomly distributed segments of the primary structure, whose cooperative bond rotation (crankshaft motion) creates "free volume" (vf), thus enabling the polymer chain eventually to achieve new positions. For a gram of dispersed solute, v/ is the difference between the specific volume of solute (vsp) and V e x "

Uf=Vsp --Uex.

(3.34)

VIII. Temperature Dependence T is the defining parameter of both the thermodynamic and kinetic states of polysaccharide dispersions. With declining T, TI increases and vf decreases until vf= 0 when all macromolecular motion ceases and the dispersion becomes essentially "frozen" with the onset of brittleness. With increases in T, the hydrogen-bond strength decreases, a poor solvent may become good, and a good solvent may become better. The strength of the hydrophobic bond increases with T (Ben-Naim, 1980). According to the law of distribution of molecular velocities (Glasstone and Lewis, 1960), molecules in two different phases, at equilibrium, are related in translation through the Boltzman equation, stated as T/,1/~ 2 "-" exp(++_E/RT).

(3.35)

n I and n 2 are the molar concentrations in phases 1 and 2, respectively. The exponential factor exp(+_E/RT) is the Boltzman factor indicating an exponential change in motion from phase 1 to phase 2.

VIII. Temperature Dependence

55

In most polysaccharide sols, phase changes like gelation, gelatinization, and melting show an inflection on graphs of the logarithm of "q versus T -1 . The gelation temperature (Tgel) may or may not be a function of the rate of cooling, which is apparently variable at high rates (Hinton, 1950) and constant at low rates (Walter and Sherman, 1983). Some substituted celluloses display anomalous behavior in that qq is directly proportional to T,

to Tgel. Heating results in an apparently p e r m a n e n t loss of "q without b o n d rupture in guar and CMC (Rao et a l . , 1981). H e n d e r s o n (1988), who observed the lowering of Tgel in the methylcellulose dispersion with increases in c i , concluded that the thermal gelation was by dewatering due to hydrophobic bonding of the methyl groups. The rubbery, sol-gel transition of an amorphous polymer eventuates over a narrow interval rather than at a single temperature. The midpoint of this interval is the glass transition temperature Tg (Kaelble, 1971; Cowie, 1991; Levine and Slade, 1992), where v f = 0. A perfectly crystalline polymer does not possess a T g ; instead, it becomes completely ordered or disordered at a precise critical t e m p e r a t u r e m t h e melting temperature T m . Melting is described as a first-order transition, because of the exactness of the heat of fusion. T m is a state property. Naturally occurring, extracted microcrystallites are imperfect as a result of their matrix associations with a miscellany of other molecules, and therefore do not show a T m , but more accurately show a softening temperature range variable with c i , the DP, prior history, rate of cooling, crystal-size homogeneity, and water of hydration. T m is located at higher temperatures for higher ci and is constant at high DP. A single-bonded linear polymer experiences complete freedom of vibrational and rotational motion above T m where the flow response is purely viscous. The greater flexibility of a r a n d o m coil causes its T m to be lower than for other conformations. From T m to T g , long segments may "freeze," whereas short segments remain m o b i l e - - a characteristic of viscoelasticity. For all these natural polymers, Tg is path-dependent and T m > T g . F r o m absolute zero (0~ to 25~ most hydrophilic solute remains separated in water to an u p p e r critical solution or u p p e r consolute temperature ( T c) (Glasstone and Lewis, 1963) w h e r e u p o n they merge. In the opposite direction (from high to low temperature), solute and solvent or two solute phases in a c o m m o n solvent may remain separated to a lower T c, where they again merge. Many cellulose derivatives have a lower Tc in the vicinity of 45~ The lower and u p p e r Tc are called cloud points because of the incipient cloudiness observed there. This incipient cloudiness in a formerly translucent dispersion is evidence that the solute has e m e r g e d from a secondary m i n i m u m on its way to a gel (Walstra et a l . , 1991). Cloudiness may be induced at a constant T by nonsolvent and electrolyte additions. Electrolyte criticality is 10-100 times more effective from

56

3. State- and Path-DependentProperties

m o n o - t o trivalence. The counterion dependency is known as the SchulzHardy rule. At a constant cation monovalence, the flocculation value (a critical m i n i m u m concentration of electrolyte usually in millimoles per liter) varies directly with the hydration radius. Arranged in decreasing order of effectiveness, the flocculating power of food electrolytes is Mg2+> Ca2+> N a + > K + (the Hofmeister or lyotropic series). The correlation with anions is less certain, but for a c o m m o n cation, O H - > C I - > -OSO~- (Jirgensons and Straumanis, 1962).

IX.

Rheology

At 1-atm pressure in the surroundings, polysaccharide deformation and flow are normally initiated either by gravity or an applied shear rate (q/); solvent (water) only flows under temperature (T) and concentration (c i) gradients. When ~qi is constant or i n d e p e n d e n t of the rate of shear (~/in s -a) or stress ('r), the flow is Newtonian. Very dilute polysaccharide dispersions are characterized mostly by Newtonian flow. At moderate concentrations, ~qi may decrease (shear-thinning; synonymous with pseudoplastic) or increase (shear-thickening; synonymous with dilatant) nonlinearly with ~/: for these dispersions, Tli is replaced with Tla (the apparent viscosity). Low DP and uniform distribution of subsdtuents are conducive to ~qi; high DP and n o n u n i f o r m distribution are conducive to +qa. A high ~1~ is believed to elicit the h u m a n oral sensation of "thickness." In shear-thinning fluids at constant T, ~qa decreases with increases in ~/ and the tertiary structure is instantaneously and completely recoverable u p o n return to ~/= 0. At very low values of ~/, shear-thinning fluids exhibit constant ~q~ (zero-shear viscosity). Shear-thinning can also occur u n d e r constant "r with the passage of time. ~/ has the opposite effect on shear-thickening fluids. A shear-thinning fluid is thixotropic if the declining 91~ slowly returns to its initial value after ~/ has ceased. Similarly, the initial structure returns to a shear-thickening fluid after relief from ~/. Structural heterogeneity contributes to thixotropy, because the dispersed particles are able to interlock into semipermanent structures. The slower and faster rates of ~qa changes in shear-thinning and shearthickening fluids result from molecular disentanglements and entanglements, respectively, wrought sometimes by slow stirring (low ~/), which in turn creates a lower or higher resistance to transport, depending on the effect of q/ on the conformation, qqE may also be shear-thinning or shearthickening. Shear-thickening fluids present difficulty in passing through tubes, orifices, and nozzles. Rheopectic (antithixotropic) fluids are shear-thickening fluids whose qq~ increases with time under constant or low ~/. Rheopexy is a property of linear

IX. Rheology

57

macromolecules---rarely of polysaccharidesmthat is readily observed in lyophobic systems. Plastic flow (unrelated to pseudoplasticity) is a linear response to "r after a critical "r (the yield point "r0) has been exceeded. A plastic fluid is synonymously called a Bingham body. A high ~/or "r may cause a uniaxial orientation of random-coil molecules in the direction of flow, resulting in temporary flow birefringence as the primary linear structure uncurls. In every unit volume, there is a ~0 created by the v o l u m e - t e m p e r a t u r e - p r e s s u r e interplay expressed in Eq. (3.1). It is the uncurling resulting from ~/, "r, and T (Fig. 2) that causes streaming potential and lowers the magnitude of ~1 and ~qa. ~li and "qa of ionic polysaccharides can be influenced by the order of addition of components, as illustrated in Fig. 3. Tli and "% are m a x i m u m when the dispersed solute is allowed to hydrate fully before the electrolyte is added. An electroviscous effect is observed as an abnormally high xli and ~1~ at very dilute concentrations (Fig. 4). As c i increases, ionization is depressed and the electroviscous effect disappears; afterward, 'lqi VS C i is that of neutral molecules (Pals and Hermans, 1952). Everett (1988) offered three reasons for electroviscositymthe distortion of the electric double layer during shear (the primary effect), double-layer repulsion between particles (the secondary effect), and a tertiary effect related to the hydrocolloidal diameter. The elimination of electroviscosity can reasonably be explained by curling, because the initial primary structure then becomes less expansive in an excess of the equivalent weight of counterions. Some polymers like CMC display an opposite electroviscous effect; they increase in 'qsp/C as NaC1 is

Figure 2 Illustration of the defibrillation of a polysaccharide double helix as a function of temperature (T) in a unit volume of solvent (water), flowing under shear rate (~/) and pressure (~). Elongation of the single helices exposes a smaller cross-sectional area, resulting in birefringence and a lower circumferential resistance to flow (lower ~1).As a result of defibrillation (e.g., doubling of the microfibrils), number-average but not weight-average properties increase.

58

3. State- and Path-Dependent Properties

0.60

-

0.50 -

0.40

o o o 0~

.,oG = = ~

-

.9.0 i

~ "o

/

0.30 -

....-.-r"

0.20 -

0.10 -

0.00

I

0.25

I

0.50

I

0.75

I

1.00

I

1.25

Concentration Figure 3 Viscosity profile of a 0.05% CMC sol, showing the effect on viscosity of order of addition of tartaric acid (TA) to water (a) before and (b) after dispersion of CMC.

added initially and then gradually decrease u p o n further additions (Dautzenberg et al., 1994). A viscoelastic fluid has the appearance of a solid body; it deforms and wholly recovers below "r0 , and only partly recovers above "r0 . From 0 to "r0 , the fluid undergoes an elastic conformational transition; above "r0 , the fluid undergoes an irreversible transition, whence the mass begins to flow toward a new equilibrium position. Carrageenan-water-polyol systems have been suggested to be industrially useful in consideration of their significant ~0 followed by shear-thinning (Tye, 1988). Viscoelasticity has advantages: for example, when butter and fruit jellies are spread over toast, they remain in place after the spreading force has been withdrawn. Paint is easily spread over a surface by the streaking of a paint brush, and remains in place when the streaking ceases. Strain hardening is an abrupt, positive deviation of xlE from the T r o u t o n rule as E increases with time. In the view of Hwang and Kokini (1991), with reference to polysaccharides, this p h e n o m e n o n is due to branch points acting as hooks, thereby increasing the resistance to flow.

X. Variable-Path Processes

59

VISCOSITY FUNCTION

p-.p-

I

p~176 p-.p-

I

I

I

I

I

CONCENTRATION

Figure 4 Typical viscosity response of a polysaccharide polyanion and a neutral molecule to concentration, showing electroviscosity in a dilute dispersion of the polyanion (negative slope segment) and linearity resulting from interactions and cancellation of electroviscosity (positive slope). P - represents the polyanion and po represents its neutral counterpart.

X. Variable-Path Processes Path-dependent properties are a function of the sequencing of the transition steps from an initial to a final state, and it is only when each event is enacted in an identical way that the integrals of path-dependent functions are themselves identical. Different polysaccharide variable-path processes were observed in a pectin-sugar-water-acid mixture dispersed at 105~ then cooled to 25~ and a mixture dispersed at 50~ then cooled to 25~ The higher-temperature gel was relatively stable, but the lower-temperature gel was unstable (Walter and Sherman, 1986). Heated agar sols gel when cooled to approximately 30~ and they remain dimensionally stable to a reheating temperature of 85~ due to relatively permanent physical crosslinks below T m (Lips et al., 1988). The reheated gels follow a hysteretic pathway to melting at the

60

3. State- and Path-Dependent Properties

higher temperature. Evaporation of a dilute polysaccharide sol to a film results in smaller pores in the film than if the pores were created by hydration of a xerogel. This sorption hysteresis results from the high volume of evaporating water that initially leaves large pores, until they contract in the xerogel, as its pore diameters attempt to narrow. In the reverse process, pores in a xerogel, made small by contraction, expand in size to accommodate the large volume of hydration water. Most polysaccharide xerogels are amorphous and consequently easily rehydratable.

A. Sols, Gels, and Pastes All polysaccharides gel, albeit under different conditions. The designation of a polysaccharide as gelling or nongelling is merely a reflection of the ability of its aqueous dispersion to solidify under the prevailing conditions of food processing and preparation. Most food gels are made by heating and cooling a sol. Curdlan and konjac gums gel by neutralization of their alkaline solutions (Kanzawa et al., 1989). Conformational transitions invariably accompany solation-gelation (Clark and Ross-Murphy, 1987; Rees et al., 1982). The polysaccharide concentration in food gels seldom amounts to more than fractions of one percent, but enough solute-solute interactions initiate contact sites or junction zones that effectively crosslink such a small amount into an infinite (Flory, 1953) cooperative (Jeffrey and Lewis, 1978) network. The junction zones are nonequilibrium assemblies (Rees, 1969), dynamic and finite in size (Oakenfull, 1991) and constructed with lengths of about 600 atoms (Sperling, 1986) into localized regions of order whose lifetimes are transient around a constant mean. It is the instantaneous averaging of these lifetimes that gives a polysaccharide gel a macroscopic picture of uniformity and permanence. The uneven distribution of junction zones leads to crosslinking inefficiencies (Kulicke and Nottelmann, 1989) that in turn lead to structural inhomogeneities and uneven texture. The chain flexibility of random coils facilitates entanglement, crosslinking, and junction-zone formation. Microcrystalline regions are also sites of junction-zone formation, but by a different mechanism (parallelism). Starch gels are occasionally composites of gelatinized granules embedded in a crystalline amylose matrix (Miles et al., 1985). In excess concentration, solute adds to an elementary structure (Dea et al., 1972) until the first floc grows into a fractal aggregate confined only by the container's dimensions. Fractal aggregates are quaternary assemblies. Kulicke and Nottelmann (1989) divided gels into three generic classes, viz., physical, ionotropic, and covalent. Physical gels are held together by hydrogen bonds and molecular entanglements; they expand when hydrated and contract when dehydrated: to Silberberg (1989) they are swollen me-

X. Variable-Path Processes

61

chanical and osmotic systems possessing the cohesive properties of solids in which a balance is struck between expansion and dissolution. Tanaka (1981) views them as fluid systems given form and maintained by a network of polymer strands under the net influence of attractive and repulsive forces. The cluster theory of gelation suggests that colloidal particles "stick" to each other almost irreversibly on contact, and form various spherical, needlelike or platelike shapes when the thermal energy, and hence Brownian motion, fall below the energy of attraction (Birdi, 1993). Walstra et al. (1991) postulated that some gels result from an enthalpy-driven aggregation mechanism of small particles with a high ratio of length to thickness. Michel et al. (1984) discounted the junction-zone mechanism of gelation for high-methoxyl gels in favor of an aggregation mechanism, but not necessarily exclusive of the junction-zone mechanism. Leloup et al. (1992) proposed an infinite network model for starch in which amylose combines with amorphous regions through an intermediate transition zone; in this model, the amorphous phase of dangling chains is responsible for the hydrodynamic behavior and porosity of the gel. Physical hydrosols and hydrogels are theoretically interconvertible, with the possible exception of high-methoxyl-pectin hydrogels that do not normally revert to a sol by reheating (Walter and Sherman, 1986): the pectin contained therein was recovered by dialysis and was comparable (by ~q measurements) to the pectin before jelly formation. Ionotropic gels develop from ionizable polysaccharides and are consequently pH- and electrolyte-sensitive; their water-absorption capacity rises with increasing concentration of ionic groups (Prud'homme et al., 1989). Gelation is induced by low pH, and the gel strength arises from autocatalytic, cooperative bonding through ion mediation, whereby Ca 2+, for example, forms the first RnCOO-Ca2+-OOCRn bridge, followed sequentially by a series of like bridges between pairs of RnCOO- (Grant et al., 1973). Cooperative bonding supplements hydrogen bonds and entanglements in building gel strength and texture. A strong firm gel has a high incidence of junction zones, crosslinks, microcrystalline sites (Stipanovic and Giammatteo, 1989), and autocatalytic cooperative bonds. Ionotropic gels have been made with alginate and Ca 2+ in a cold process involving a stream of sodium alginate injected into a bath of calcium chloride (Kelco, 1986); the rate of delivery of the sol affects the final gel texture. In another process, a sparingly soluble calciumsalt was dissolved or suspended in a mixed dispersion of alginate and pectinate, and Ca 2+ was slowly generated by a lactone (Morris and Chilvers, 1984). Ionotropic polysaccharides gel also by monovalent cations, but the mechanism, presently incompletely understood, is different from autocatalytic cooperative bonding with divalent ions: the dehydration of carboxyl groups is believed to be involved somehow, and K + and Na + appear to behave differently in structure ordering and disordering (Miyoshi et al., 1994).

62

3. State- and Path-Dependent Properties

Ionotropic gels are more acid-stable than the sols (Guiseley et al., 1980) because of protonation of the electrolyte-sensitive acidic groups and immobilization of the molecules in the network. Covalent gels develop from copolymerization with bifuncfional crosslinkers: these are industrial-purpose gels having little or no relevance to food, except as aids in processing and research. Chandrasekaran et al. (1988a) proposed Rees' mechanism (Rees, 1969) that random coils above T m form double-helical junction zones upon cooling, which then aggregate prior to gelafion. By themselves, the double helices do not cause gelafion, but coordination complexes are required to be built with hydrated m o n o - o r divalent cations (Chandrasekaran et al., 1988c). Proof that double-helical junction zones alone did not cause gelafion lay in the fact that ~-carrageenan in the presence of Li § formed double helices but did not aggregate and hence did not gel (Morris et al., 1980). Kanzawa et al. (1989) found that the width of microfibrils (50-250/~) had an influence on gel structure, in contrast with viscous, nongelling dispersions in which shorter dimensions (10-20 A) predominated. Beet pectin and wheat-flour pentosans undergo an oxidative gelafion (Neukom, 1976; Neukom and Markwalder, 1978) instigated by ferulic acid peroxidase through diferulic acid crosslinking. Crowe (1989) and Thibault et al. (1991) effected the same crosslinking with persulfate and chlorine, respectively. Polysaccharide pastes are concentrated dispersions and suspensions with vastly diminished Brownian activity; they differ from gels by failing to undergo the liquid-solid (coil-helix) transition.

B. Emulsions and Foams

Polysaccharides may exercise a protective action in an emulsion and foam as a thin film at liquid-liquid (emulsion) and liquid-air (foam) interfaces. The hydrophile-lipophile balance in the macromolecules as well as +i determines whether or not the emulsion is an oil-in-water or water-in-oil dispersion (Vold and Vold, 1983; Dickinson, 1992).

C. Xerogels and Films

The anhydrous solid phase remaining after an apparently complete evaporation of water from a polysaccharide hydrosol is a polysaccharide xerogel, capable of retaining 20-30% water, yet appearing to be dry. Difficulty in removing residual water from polysaccharide xerogels makes a condition of 0% water virtually impossible. Dilute electrolytes initially present in the sol become concentrated as a result of the evaporation of water, leading to

X. Variable-Path Processes

63

localized regions of high i with ramifications for the electrokinetics of the xerogel. Many young fruits and vegetables are polysaccharide hydrosols, but convert to xerogels during senescence. Upon rehydration, a xerogel is capable of swelling to 10-100 times its dry volume without disintegrating. Such inordinate volume expansion is the reason why dehydrated fruit and vegetable food preparations require so much more liquid than their weight would otherwise suggest. Films are two-dimensional xerogels. Composite polysaccharide films contain complementary cosolutes, each designed to fulfil one or more shortcomings of the other(s). Polysaccharide films have a low permeability to oxygen, a high permeability to moisture, and low tensile strength. The permeability is sensitive to the number and distribution of segments (Silberberg, 1992), i.e., to the number and distribution of junction zones, because these block the solvent's path. In other words, permeability is highestwhere junction zones are least dense. According to Kester and Fennema (1986), a high-moisture gelatinous polysaccharide acts more as a short-life "sacrificing agent" than as a moisture barrier, inasmuch as there is a preferential release of moisture from the moisture-laden coating to that of the packaged item. Films with triple helices are apparently stronger than films without (Schulz et al., 1992). Polysaccharide films are used to protect the delicate flavor and aroma of fruits and vegetables from a deleterious physical environment. Cellulose xerogels do not readily rehydrate because of the strong hydrogen bonding of the microfibrils in the dry state, but they make excellent protective membranes and coatings, relying on the tensile and compressive strengths of their interwoven microfibrils and on plasticizers (e.g., glycerol and dextrin) to overcome their characteristic brittleness and hardness. Non-free-draining water has a minor plasticizing effect on polysaccharide films. Lipid plasticizers increase hydrophobicity and decrease the rate of moisture transport across the film. Thermosetting cellulose films obstruct lipid migration into frying foods (Nelson and Fennema, 1991). Engineering cellulose films for specialized properties (high solute retention, high solvent exchange, etc.) requires control of a number of variables (Shen and Cabasso, 1982).

D. Aerosols

An aerosol is a dispersion of discrete particles in a stream of gas. Starch and cellulose aerosols are potential fire hazards in granaries where friction between the moving, micronized particles causes electrification, whereupon separate accumulations of positive and negative charges may discharge as an electric spark and ignite the combustible solute (contact electrification; synonymous with triboelectrification; Ross and Morrison, 1988).

64

3. State- and Path-Dependent Properties

E. Suspensions Suspensions are macroscopic, heterogeneous, usually liquid-solid systems that contain all or a fraction of solute larger than colloidal dimensions. A polysaccharide suspension may consist of a continuous network of irregularly shaped particles (Walstra et al., 1991). In these multicomponent systems, particles dissimilar in size diffuse at dissimilar rates. The large fraction refracts and the colloidal fraction scatters light. In number- and weight-average measurements, the multicomponent phase properties are indistinguishable from those of a homogeneous phase, but they have much larger variances. Examples of polysaccharide food suspensions are tomato ketchup, unclarified fruit juices and beverages, fruit pulp, and chocolate milk.

Xl. Stability and Instability To the food processor and preparer, a stable polysaccharide dispersion is one that has a long shelf-life. For maximum utilitarian benefit, most dispersions are maintained in a kinetically stable state instead of a thermodynamically stable state. Absolute thermodynamic equilibrium is the existence of the solute in one spherical mass at the bottom of a container where each component of a dispersion reverts to its ground-state energy level. Thermodynamic stability is thus antithetical to superior food quality: it is mostly observed between macromolecules with a different chemical composition and those with the capacity to form only weak bonds with each other (Tolstoguzov, 1993). Kinetic stability requires the particle phase to remain dispersed in prolonged metastable equilibrium under a prescribed set of conditions. By thixotropy, the life of a suspension of discrete, heterogeneously shaped particles in an aqueous medium is prolonged over that of homogeneously shaped particles, because the semipermanent structure they create takes a longer time to dehydrate and phase-separate spontaneously before settling. Destabilization is the process of a dispersion's progression from kinetic to thermodynamic equilibrium (see Chapter 7, Fig. 5). Myers (1960) recognized two kinds of stability, viz., inherent stability as a function of time, and induced stability as a function of stimuli. Induced kinetic stability necessitates the expenditure of work exceeding ~.12 Acceptance of a stabilization role for water has not been universal; Jirgensons (1946) argued against it, enunciating instead the influence of particle shape and the mutual chemical affinity between atomic groups at the solid-liquid interface. 12. The unit of a potential is the joule (force times distance per unit charge).

XI. Stability and Instability

65

By the logic of the capacitor model, "salting in" is a stabilizing mechanism whereby ~ is lowered and in turn Q / ~ is elevated. The capacitor mo.del does not explain "salting out" as easily as does the electrolyte effect of i directly on the Debye length. Steric stabilization differs from electrostatic stabilization in not being a function of a net force, but of the thickness of an adsorbed layer. When +i equals 5-10%, stabilizing and destabilizing forces extend beyond the length of the electrostatic, interparticle barrier (Cabane et al., 1989). At this distance, attraction and repulsion are inconsequential, and electrolytes therefore have little effect. Bergenstahl (1988) proposed that the steric stabilization of emulsions by gums in the presence of a surfactant involves adsorption of the gum on the surfactant to form a combined structure constituted by a primary surfactant layer covered by an adsorbed polymer layer. Destabilization is signalled by incipient flocculation. The latter occurrence was considered by Vold and Vold (1983) to be the initial reversible stage of aggregation. Coalescence and coagulation are qualitatively synonymous terms. Hiemenz (1986) made a distinction by considering flocculation as a process that allows small particles to retain their identity but lose their kinetic independence, and by considering coalescence as a loss of particle identity in favor of larger particles. A floc is less dense, because it occludes more water than a coagulum. Gelation, precipitation, and crystallization are stages of one continuum in a sol between dispersion and deposition. Small quantities of polysaccharides can flocculate a dispersed phase through bridging (Ward-Smith, et al., 1994), whereby one attached molecule with other adsorption sites along it may attach itself to another or more surfaces, acting as the "bridge"; this p h e n o m e n o n is called bridging flocculation. A bridge may instead cause steric stabilization of the dispersed phase. In the view of van Oss (1991), steric stabilization is predominantly a polar repulsion between macromolecules that is influenced not by Brownian activity, but by osmosis. Depletion flocculation arises when a large unadsorbed, flocculating cosolute molecule does not fit properly into a small interparticle volume at the interface and the cosolute molecule accompanied by solvent is consequently expelled from the interface. As a result, the interparticle distance is shortened, causing an approach to x e and flocculation. Depletion stabilization is possible if the particle-cosolute attraction is greater than the particle-particle or cosolute-cosolute attraction. Cations flocculate hydrosols at critical ionic strengths that vary with polymer concentration, particle size, temperature, etc. For mono-, di-, and tervalent cations, in ascending order of strength, the critical concentration is 1, 0.03, and 0.001 m m o l / L (Vold and Vold, 1983). In the presence of flocculating cations, during the progression from kinetic to thermodynamic stability, dispersed polysaccharides gradually lose mobility ( - A S ) , their surfaces merge, the particles grow larger and are fewer.

66

3. State- and Path-Dependent Properties

TABLE II Conditions and Mechanisms of Stability of Polysaccharide Sols

Condition

Mechanism

Micronization (Homogenization a)

Buoyancy Brownian motion

Hydration

Capacitance Maximum excluded volume Minimum excluded volume effects Coil volume expansion

Dilution

Capacitance Maximum excluded volume Minimum excluded volume Coil volume expansion Salting in

Heating

Brownian motion ( + A H , +AS)

Low acidity

Dissociation Electroviscosity potential

Neutralization

Soluble salt formation

Cosolute addition

Protective colloid action Steric stabilization

Viscosity increase

Slow sedimentation rate

aIn this context, homogenization refers to reduction of particle size by passage of the disperse system through a small aperture under pressure.

Undesirable emulsions and foams in food-processing operations are " b r o k e n " by antifoaming agents whose exact mechanisms of action are uncertain, although macromolecules performing this function are known to create ordered assemblies at the interface and are themselves excellent emulsifiers. Antifoaming agents thin a n d weaken small regions of an adsorbed film (Shaw, 1992); all rapidly lessen or0,i to the extent that the attractive forces between the antifoam and one of the phases (adhesion) exceed the attractive forces between like molecules (cohesion). Creaming is the opposite of sedimentation; it occurs when the solute phase (usually oil) has a density less than that of water. The conditions and mode of action that contribute to the kinetic stability of polysaccharide sols are listed in Table II.

A. Aging and Phase Separation In polysaccharide dispersions, a constant-temperature, constant-pressure separation of any or all of the solute f r o m its dispersion m e d i u m may be effected spontaneously by time (aging) or may be actuated by external

XI. Stability and Instability

67

stimuli (induction). Concomitantly, viscosity parameters Rg and + are lowered. During the process, smaller-size solute particles tend to grow into larger-size solute particlesma p h e n o m e n o n termed Ostwald ripening. High DP, solute and electrolyte concentrations, low temperatures, nonsolvents, evaporation, storage-temperature fluctuations, and stimuli as innocuous and unobserved as vibrations in the surroundings accelerate either process. The purpose of a protective colloid is to extend the duration (shelf-life) of the apparent monophase. In either spontaneous or induced destabilization, the solute is transformed from its initially high AE to a final low AE state.

B. Coacervation Coacervation is the separation into two liquid phases of a ternary dispersion, each phase containing a preponderance of one solute and a minor concentration of the other, and vice versa: each phase is a coacervate. The event is simple coacervation if the cosolutes have identical charge; it is complex coacervation if the cosolutes are oppositely charged (Jirgensons and Straumanis, 1962). Either phase may develop a network independently of the other (Moritaka et al., 1980), or one phase may be suspended as droplets in the other. Alternatively, one solvent-depleted phase may contain the two cosolutes, while the other phase is preponderantly solvent.

C. Syneresis Syneresis is the tendency of gels to release spontaneously small volumes of liquid, occasioned by the rupture of weak bonds, under an internal -r. This mechanism lowers + AE (Walter, 1991), as the gel attempts to return its components to their respective ground states. Rigid gels are prone to synerize, because the elastic component does not possess the mobility of the viscous component and, consequently, phase separation is the only energyreleasing alternative to viscous flow. Soft gels synerize when "r exceeds the bonding strength. Syneresis is sometimes accelerated by freeze-thaw cycles. Starch and pectin gels are noted for their ability to synerize; xanthan has received wide acclaim as a syneresis-controlling polysaccharide (Rocks, 1971); K-carrageenan gels are firm, brittle, and given to syneresis, whereas ~-carrageenan gels are soft, elastic, and syneresis-free (Roesen, 1992). Corn starch all but eliminated syneresis in a 4% curdlan gel that had been subjected to freezing and thawing (Nakao et al., 1991); one possible explanation of the amelioration is that corn starch is a humectant. Other syneresis-controlling practices include pH and soluble-solids adjustment (Konno et al., 1979; Hercules Inc., 1985). In pectin jellies in which the defect is frequently observed, syneresis may be avoided by simply changing the sugar concentration or the pectin (Hercules Inc., 1985).

68

3. State- and Path-Dependent Properties

D. Sedimentation A sediment is a solid phase separated from its dispersion medium in a relatively solvent-free condition: the process is called sedimentation or deposition. The rate of sedimentation depends on ~qo, +i, Mi, particle size, and the density difference between the solvent and solute (Scholte, 1975; Harding et al., 1991a). The density of highly hydrated particles is approximately equal to the density of water: a large volume of non-free-draining water may therefore cause a floc to remain suspended almost indefinitely. Very small density differences do not provide enough of a gradient to affect rapid deposition. A polyanion's sediment layer is more diffuse than that of a neutral polysaccharide, because of interparticle carboxyl-charge repulsion. Easy repeptization of uronan-containing sediment in juices and wines presents difficulty during filtration and decantation.

E. Encapsulation Polymers may be induced to encapsulate other molecules by a variety of means (Risch and Reineccius, 1995) as diverse as dipping, spray-drying, extrusion, evaporation, and coacervation: each t e c h n i q u e has its special applications, strengths, and weaknesses. Advantages in common are the protection and slow release of the encapsulate. In any of the mechanisms, a coagulable polymer precipitates around a core of labile material. Polysaccharides are regular encapsulating polymers (Risch and Reineccius, 1995); acacia gum is particularly efficacious because of its protein content. In a microencapsulation method, the encapsulatemusually an oil, flavor, enzyme, or medicinalmis emulsified in a dilute aqueous gelatin sol, a polysaccharide is added, and conditions are adjusted to favor coacervation. The encapsulate should not be truly soluble in the solvent or the cosolutes and the cosolutes should be differentially soluble in the liquid solvent. As much as 60-98% of the labile substance may be harvested by microencapsulation to yield microcapsules in the form of a free-flowing powder (Sirine, 1968). In a spray-drying method of encapsulation, Zhao and Whistler (1994) suspended starch to a concentration of 30% in water containing 0.1-1.0% gelatin or any of a number of polysaccharide bonding agents: the suspension was forced through an orifice (2 mm diameter) under 80-100 psig at an inlet temperature of 120~ and outlet temperature of 76~ Porous 10-40-nmdiameter spherical capsules were obtained that were then immersed in peppermint oil. After diffusion of the peppermint oil into the capsules, the spheres were rinsed free of oil and coated in a fluidized bed with a 3%

XII. Summary

69

dispersion of the bonding polysaccharide. The finished capsules contained 33-48% peppermint oil. What Tye (1988) called "entrapping technology" involves dropwise gelation in a KC1 bath of an emulsified solute and carrageenan (1% in distilled water). The dried (and presumably washed) gel capsules were reportedly capable of retaining in the carrageenan network any dissolved or emulsified cosolute.

Xll. Summary Macromolecular conformations and reversible order-disorder and disord e r - o r d e r transitions are highly sensitive to solvent, temperature, pressure, pH, water activity, and metal ions. Polyanions are distinguished from neutral molecules by their sensitivity to electrolytes. Whereas synthetic polymers do not normally dissolve or disperse spontaneously, some polysaccharides may do so in water (hydration), given their strong hydrophilicity. The random coil is a high-energy conformation and the helix is a relatively low-energy conformation: the former is disordered and the latter is ordered. The coil-helix transition is consequently attended with a large negative entropy change and a larger negative enthalpy change. The energy status of a final product depends on the treatments it underwent during the final stages of processing and preparation. High-energy food dispersions require special treatment for prolongation of their shelf-life: examples of such treatments are the inclusion of a protective colloid and maintaining high solvent viscosity. Polysaccharide dispersions phase-separate spontaneously, a p h e n o m e n o n called aging. Phase-separation may be induced in special systems, under controlled conditions (e.g., encapsulation), to industrial and commercial advantage.

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CHAPTER 4

Concentration Regimes and Mathematical Modeling I. Introduction Polysaccharide uses span dilute, semidilute, and concentrated regimes, from fractions of a percent in a hydrosol to higher than 90% in a xerogel. At the lower limit, solute-solute interactions are m i n i m u m if not entirely eliminated, and a macroscopic property (P) is theoretically the sum of the properties of independently acting molecules. At the u p p e r limit, P is the combined property of multiples of molecules seen as clusters, each cluster acting as a hydrodynamic unit. A critical micelle concentration (c*, Fig. 1) differentiates the additive response of single molecules from that of clusters. When the n u m b e r (n i) of equivalent spheres each of weight wi increases in a volume of water (Vo), the solution density n i w i / V i increases until the minim u m x e is reached when the dispersion m e d i u m becomes saturated, c* depends intensively on the DP and extensively on n i w i, u n d e r the j o i n t influence of conformation, cosolutes, nonsolvents, T, ~/, and "r. High M polysaccharides are less energetic and have a lower c* than do low M polysaccharides, causing the molecules to cluster or precipitate sooner.

II. Concentration Regimes There are four to six concentration regimes (Dautzenberg et al., 1994). Edwards (1966) classified them into three broad types on the basis of the n u m b e r of polymer chains present, the n u m b e r and length of the m o n o m e r constituting them, and the volume of the m o n o m e r relative to the volume V/. At use levels as low as parts per thousand in food, no one definition of "dilute," "semidilute," or " c o n c e n t r a t e d " encompasses the range of weight-volume or volume-volume concentrations necessary to elicit a par-

71

72

4. Concentration Regimes and Mathematical Modeling

14.0

12.0

10.0

TI

i

j*

8.0

6.0 J

,,,

)

I,,,

o. 10

l

I

0.20

i

I

CMC

,

c 0.30

I

0.40

i

I

0.50

c(g/lO2ml) Figure I Viscosity-concentration (xl vs c) profile at 26~ showing the critical micelle concentration (c*).

of an aqueous pectin dispersion

ticular physical response or h u m a n sensation: for example, a constant weight each of high-methoxyl and low-methoxyl pectin with the same DP gels u n d e r different conditions involving water, sugar, acid, calcium, and heat. Without acid, a heated high-methoxyl-pectin jelly formula remains a sol, but with acid, it transforms to a gel. Gellan gum, requiring no sugar or acid, gels at 0.05% concentration in water, whereas 0.5-0.8% pectin requires sugar and acid. Similarly, pectic acid, an ionic polysaccharide, and dextrin, a neutral polysaccharide, make very thin fluids at 2% concentration, but CMC and guar gum, also an ionic and a neutral polysaccharide, respectively, make extremely viscous fluids at one-tenth that amount. The singular event comm o n to the three gels is a high incidence of solute-solute contacts.

A. The Dilute Regime Doi and Edwards (1986) characterized a dilute solution as one of sufficiently low concentration that the polymer molecules are separated from each other. Dilute sols are normally characterized by a linear d e p e n d e n c e of P on c i and often by Newtonian flow. A constant rate of change of ~i vs ci is

II. Concentration Regimes

73

desirable for uniform fluid texture in liquid foods like dietetic beverages and clarified fruit juices, and for uniform delivery in flavor-release capsules (Baines and Morris, 1988) where slow release is essential. Linearity holds for neutral polysaccharides in water and for ionic polysaccharides in an excess of electrolytes. Dilute dispersions have a very small +i and, especially in the case of polyelectrolytes, a dielectric constant that is unfavorable for cluster formation (Eisenberg and King, 1977). Dilution may not eliminate the structuring effected by polysaccharides in water, because structuring does indeed exist in " t h i n " liquids (Schenz and Fugitt, 1992).

B. The Concentrated Regime At the upper limit of the ci range, vf decreases to a minimum as the molecules are progressively immobilized, effectively making "good" and " p o o r " solvents functionally indistinguishable. In this regime, viscosity merges into elasticity, P becomes independent of ci, and the dispersion simulates the behavior of a molten polymer.

C. The Semidilute Regime The vicinity of c* is transitional to the dilute and concentrated regimes across a narrow ci range where the topologically linear but conformationally distorted coils touch, overlap, and entangle, to the extent that free movement of segments is restricted and diffusion is unidirectional (reptation), because of the constraints on radial motion imposed by neighboring segments. Reptation was theorized by de Gennes (1979) and experimentally proven by Russell et al. (1993). Cluster formation and complex flow originate in entanglements in this region. A careful study of xli vs c i (for some polysaccharides) reveals an inflection closely preceding c* where, it is claimed, molecules actually begin to entangle (Launay et al., 1986). In the transition zone of c*, P is scaled to a power function of c / c * (Dautzenberg et al., 1994). The contact points are dynamic and identical, except that they have much shorter lifetimes below c* than above. Reversible gelation is ascribed to few entanglements of short-life contact points with insufficient combined strength and duration to maintain a suprastructure indefinitely, unlike those in an irreversible gel. Numerous contact points encumber diffusion. Random-coil polysaccharides make strong films, because they are given to a high incidence of long-life contact points; for the same reason, they are good carriers of flavor. Notably, concentration had no effect on flavor release from a nongelling xanthan dispersion (Baines and Morris, 1988).

74

4. Concentration Regimesand Mathematical Modeling

Time-dependent flow and viscoelasticity begin to be evidenced in the semidilute regime where +i relative to +i in the dilute regime is high, the dispersion remains sensitive to T, and P is equally a function of micellization as of colligative action. Pseudoplasticity was observed in 0.5-2.0% guar gum and 0.5-1.2% xanthan gum; thixotropy was observed in 0.8-1.2% furcellaran (Rao and Kenny, 1975). Okiyama et al. (1993) observed dilatancy at a low q/in an aqueous 0.7% dispersion of a disintegrated bacterial polysaccharide. Some gums (e.g., CMC and guar) change from dilatant to plastic flow (Balmaceda et al., 1973). Thixotropic fluids are easily mistaken for pastes and gels, due to their self-assembly into suprastructures at rest. The dimensionless product c[~q] is defined as the coil overlap parameter; it provides information about the changing nature of the interactions in a dispersion (Blanshard and Mitchell, 1979; Morris et al., 1981). For dilute dispersions, i.e., below c*, the slope of log('qsp/C i) vs log(c[qq]) universally approximates 1.4. At the upper practical extreme, with exceptions (especially the galactomannans; Morris et al., 1981), the slope increases sharply to 3.3, illustrating wide deviations from Newtonian flow in the segment approaching elasticity. The deviations are significant when 5 < ci[~q] < 10 (Barnes et al., 1989).

III. M a t h e m a t i c a l

Modeling

P is characterized mathematically as a scalar variable (v) raised to the magnitude of one or more systemic constants. The defining equation may be a linear ( P = my + P), a quadratic ( P = j v 2 + j ' v +j"), an exponential ( P = Kev + K'), or a power (P = k v ~ + co) function of v, where m, P, j, j', j", co, K, K' and a are algebraic constants. Assisted by these models, the concept of the equivalent hydrodynamic sphere, under 0 conditions, facilitates characterizations of properties and calculations of various size parameters. Many aqueous polysaccharide properties are a function of M to an asymptotic limit. Linear flow is readily amenable to mathematical analysis and, not surprisingly, a number of equations to measure ~q have thus been derived. The Maxwell and Voigt-Kelvin models (Kaelble, 1971; V. N. M. Rao, 1992) combine mechanics and mathematics to demonstrate that viscoelasticity and elasticity are mechanisms of storage and dissipation of AE. Although ~q is a simple property to measure, rheological data can nevertheless be difficult to interpret (Barker and Grimson, 1991). In contemporary methodologies, sizeand shape-related complexities have largely been minimized, where possible, by extrapolation to infinite dilution (c i = 0).

i11. Mathematical Modeling

75

A. The Stokes Equation In 1851, Stokes derived Eq. (4.1) from the model of solid spherical particles falling independently through a homogeneous liquid without Brownian motion, slippage, and wall effects. Slippage is an inconstant rate of fall; wall effects refer to axial orientation in the outermost planes of fluid in contact with a surface, and the differential velocity of flow in the outermost and innermost planes of a fluid in a confining tube: O x / O t = k ( d s - do) gr2/qq0 .

(4.1)

The spheres with radius r i and density d s fall through a solvent with density d o and viscosity +q0 at a rate Ox/Ot; g is gravity. F, counterbalancing gravity, is equal to the product of fc and Ox/Ot, i.e., F=f~(Ox/Ot).

(4.2)

The variables in Eqs. (4.1) and (4.2) are conventionally expressed in cgs 1~ units. For a spherical geometry (Hiemenz, 1986), fc = 6pi Ti0r .

(4.3)

Equations (4.2) and (4.3) show that molecularly h o m o g e n e o u s solute providing a larger fc settles more slowly than does solute providing smaller fc. Alternatively stated, larger particles settle more slowly than smaller particles with the same density, barring hydration. Other empirical offshoots from the Stokes law were attempted, but complications arose from an initial lack of awareness of the contributions of hydration to particle factors (Mehl et al., 1940).

B. The Poiseuille Equation V/ flowing under "r in t i seconds through a cylindrical tube of radius r and length 1 is inversely proportional to Tli and the fourth power of r: Vi = kr 4t i ' r / ( 'flit ) .

(4.4)

The dimensions of 'lfli are mass per unit distance per unit time, which, in cgs terminology when "r is in dynes (g cm s-Z), converts to poise (g cm-1 s-1) (Appendix 1). In inks 14 terminology, F is in newtons (kg m s -2) and the ~li dimensions cancel to kilograms per meter per second (kg m -1 s-l). One 13. Abbreviation for centimeter-gram-second. 14. Abbreviation for meter-kilogram-second.

76

4. Concentration Regimes and Mathematical Modeling

dyne equals 10 -5 newton (N). "r expressed in newtons per meter squared is a pascal unit (Pa); 1 Pa = 1 N m -2. The SI unit of ~ is the pascal second (Pa s). A ratio of xl in cgs units and xl in mks units [(g c m - 1 s- 1)(kg m - 1 s- 1)- 1] cancels to 0.1; 1 poise = 0.1 Pa's. Equation (4.4) is an unwitting statement that the velocity ( l / t ) of a sol's planar flow is inversely proportional to "qi. A capillary viscometer is designed to maintain r, l, V, and "r (1 atm) constant, so that xli is directly proportional to t i . The generalized equation for a single m e a s u r e m e n t (single-point viscometry) is xli = k t i .

(4.5)

T h e unit of capillary ~q is the stoke, defined as poise per density, which reduces to centimeters squared per second. Having no reference to mass and force, capillary ~q is also referred to as kinematic TI. The T]i ti relationships have been formalized into a series of ~q functions (Table I). If t o and t i are m e a s u r e d for a series of dispersions at different c i and the data are plotted as 'qsp/Ci VS Ci and extrapolated to c i -- 0, the intercept is ["q] in a volume per unit weight. [~1] reflects the magnitude of the interactions between flexible molecules in energetically favorable and unfavorable solvents (Alfrey et al., 1942; Alfrey, 1947; Berth et al., 1982); it is strongly influenced by T and is. The slope of "qsp/Ci vs c i in a unit of volume squared per unit weight squared has functional significance; the steeper it is, the m o r e thickening power c o m p o n e n t i has. For convenience, the absolute percentage value of c i may hereinafter be used when a numerical r e m a i n d e r --

Ci

--

TABLE I Equations Relating a Dispersion's Flow Time (t i) and Viscosity ( q i ) to the Flow Time (t o) and Viscosity (no) of the Dispersion Medium Containing Solute in Weight per Volume Concentrations (c i) at a Constant Temperature

Viscosity parameter Relative viscosity ('qrel) Specific viscosity ('qsp) Viscosity number ('qsp//Ci) (Reduced specific viscosity) Inherent viscosity Intrinsic viscosity ([~q]) (Limiting viscosity number)

Equation ~qi/"qo = t i / t o (~qi - - ~ q o ) / ~ o = (ti - t o ) / t o ( ~ q i / ~ o ) - 1 - - ( t i / t o) - 1

[(1]i//~o ) -- 1]//Ci--[(ti/to)-

11/C i

ln(~qi/~o)/C i = ln(ti/to)/C i [(~qi/~o)-

l]/ci = [(ti/to)-

l]/ci 0

lim c i ~

[ln(~qi/~lo)]/c i = [ l n ( t i / t o ) ] / c i lim c i ~ 0

III. Mathematical Modeling

77

o f Tlsp/C i is given. Capillary ~1 data are precise and linear, once t i approximates 100 s, V / t approximates 0.1 cm ~ s -a (Jirgensons and Straumanis, 1962) and t i / t o < 2 (Allcock and Lampe, 1981). Measurements taken from a series of different c i arrived at i n s i t u by dilution in specially designed viscometers comprise the capillary viscometry technique known as dilution viscometry. Neither single-point nor dilution viscometry is suitable for suspensions, because of the unreliability of their t i resulting from heterogeneities of particle size, shape, and interaction. Variations in t i are conducive to slippage, wall effects, and turbulence. A Reynolds number, computed as the dimensionless ratio of the length of the cylinder multiplied by the flow velocity and the kinematic qq [(cm 2 s- 1)(cm 2 s- 1)- 1], is a ratio of inertial and viscous forces (Van de Ven, 1989). Newtonian flow occurs below a Reynolds n u m b e r of 2200; turbulent flow occurs above 2200. In a crude adaptation of viscometry known as Bostwick consistometry, t i is held constant and 1 is measured as a function of qqa. This technique is suitable for liquid suspensions, e.g., tomato ketchup, that need not be subjected to rigorous quality control. A different experimental design, called rotational viscometry, exploits the principle of fluid resistance, whereby concentrated dispersions and suspensions at and above c* are sheared between two surfaces moving with different velocities relative to each other at constant or variable "r. Time dependence is measurable by rotational viscometry but not by capillary viscometry.

C. The Huggins Equation Huggins (1942) derived Eq. (4.6) to characterize ~li vs c i . The frictional coefficient (k) was included to account for "the sizes, shapes and cohesional properties of long-chain, neutral molecules": qqsp = ["q]ci + Tlsp/Ci

k[

nq]2c2 ,

= [ ~1] + k[ ~1]2c i

(4.6) .

(4.7)

k, also called the Huggins interaction coefficient, is alleged to be specific to the particular solute-solvent system. Equation (4.7) with slope k[~l]2 is the linear form of Eq. (4.6). The steepest slope may sometimes betoken the poorest solvent, due to solute-solute interaction, and at other times betoken the best solvent, due to solute-solvent interaction (Alfrey, 1947). Substituting [3 for k['q] 2, the effect of ci o n Tlsp is shown by the exponents in an

78

4. Concentration Regimes and Mathematical Modeling

expansion series [Eq. (4.8)] to exaggerate small property differences:

"qsp/Ci = [~11 + f3ci + 2f3c2 + 6f3c~i + " " .

(4.8)

This exaggeration makes possible the characterization of polysaccharides by coordinate orientation (Walter, 1991). For routine purposes, accuracy to more than the squared term is seldom required. By use of the Huggins interaction coefficient, a configurational distinction was made between guar gum and locust bean gum (Elfak et al., 1977); with it, solute-solute interactions were indexed (Launay et al., 1986). A significant increase in 13 was measured for different classes of galacturonans dispersed in water containing varying amounts of ethanol, when the ethanol concentration was 15-25%: there was no reliable trend in "q or k', by itself, but the combined change ([3)was unmistakable (Walter and Sherman, 1988). The increase was attributed to the higher frequency of solute-solute contacts in the increasingly hydrophobic medium (see Fig. 2 in Chapter 7). Aging lowers [~1] significantly (Walter and Sherman, 1983). D. The Martin Equation

Equation (4.9) is a logarithmic equivalent of the Huggins equation that obscures the exaggerated effect of the exponent and yields a wider linear range:

log(%/~)

= log[~l +

k[~]c~.

(4.9)

E. The Kraemer Equation

The Kraemer equation is another logarithmic equivalent of the Huggins equation:

(log "qrel)/gi-- [TI] -~-k[TI

2 c i

9

(4.10)

The Huggins and Kraemer lines plotted in the same graph converge at [~1]. F. The Schulz-Blaschke Equation

One form of the Schulz-Blaschke equation is stated as the second-order relationship [~11 = ('qsp/Ci)/( 1 + k~lsp).

(4.11)

79

III. Mathematical Modeling

(1 + kqqsp)/'qsp is the slope of the locus 2). This equation permits a wide range man, 1975) and is the most suitable concentration (Vink, 1954), although because they may conceal nonlinearity.

delineated by [~1]-1 vs c i (Appendix of linearity (Carpenter and Westerfor computing [qq] from a single such computations are disfavored

G. The Newton Equation According to Newton's law [Eq. (4.12)], "r generated in a flowing dispersion over t is a function of ~li" "r = ~qi ~/ // t -- 1]i ~/ .

(4.12)

Newtonian fluids comply with Eq. (4.12). When ~i @ 'r/'y, Tli is replaced by ~la and ~/ must be stipulated. Plastic flow Complies with Eq. (4.13) and is normally linear after ~0" a'-a" o -- "qi~/.

(4.13)

The counterpart of ~/ in elastic fluids is the strain rate (6).

H. The Power-Law Equation The general equation for viscometry of non-Newtonian dispersions is "r = K~/v ,

(4.14)

v 4= 1. Substituting for "r from Eq. (4.12), Eq. (4.14) becomes ~1~ =k~/(v- 1).

(4.15)

k is the consistency. Shear-thinning is characterized by 0 < v < 1 and shearthickening is characterized by v > 1. Launay et a l . (1986) considered a major deficiency of Eq. (4.15) to be its prediction of infinite TI, instead of Newtonian nq, at ~/= 0. One advantage of Eq. (4.15) in rotational viscometry is that it enables the measurement of qqa and time constants of thick fluids. Over an extended range of ~/, log ~la VS log ~/ reveals two segments, one at the lower and the other at the upper Newtonian region, where [3 = 0. ~la in each segment is referred to as zero-shear nq. The rotational viscometry of polysaccharide systems begins with measurements at very low ~/in the upper Newtonian region and passes through a shear-thinning interval to the lower Newtonian region.

80

4. Concentration Regimesand Mathematical Modeling

I. Hooke's Equation The basic equation for elasticity is Hooke's law, stated as r = Ge.

(4.16)

G is the modulus of elasticity (synonymous with elastic modulus, rigidity modulus, bulk modulus, stress relaxation modulus), depending on the experimental design. G is remarkably sensitive to T; it increases from T m to Tg, depending again on the degree of ionization (Brondsted and Kopecek, 1992), and decreases with nonsolvent and electrolyte additions to a critical swelling ratio, whence it increases again upon further additions (Oppermann, 1992). The larger the G, the firmer is the gel. T h e reciprocal of G (e/'r) is the compliance. By a process called creep, a viscoelastic fluid subjected to an instantaneously applied constant "r gradually deforms, storing energy ( + AE) in the process. Upon the release of "r over a time interval (t), e slowly returns to its original shape in an attempt to cancel + AE: hence, creep is described as retarded elasticity. The recovery stage is called relaxation, which can occur over an indefinitely long interval; the longer the interval, the more elastic than viscous is a gel. In a creep test (Fig. 2a), "r is held constant until gradually increases to a m a x i m u m (Emax) characterized by the cessation of flow (dEmax/dt- 0). The creep curve demarcates three stages in a pressurized, viscoelastic f l u i d m a n initial, "r-independent, completely recoverable (elastic) response, followed by variable-rate deformation (retardation) from fast to slow when weak physical bonds rupture, and, third, terminal nonrecoverable, viscous transport. ~/'r vs t may alternatively be plotted as a creep compliance curve (Fig. 2a). In the Maxwell model, ~ and G are in series and "r varies with time, while e is held constant. In the Voigt-Kelvin model, ~qa n d G are in parallel, each exercising a damping effect on the other, and an instantly imposed "r is held constant, while e varies with time (Van Wazer et al., 1963). The Maxwell model appears to fit polysaccharides with long linear segments interrupted by junction zones, and the Voigt-Kelvin model appears to fit branched polysaccharides containing junction zones scattered throughout side chains. Emax in a Maxwell fluid is the sum of Hooke's and Newton's laws [Eqs. (4.12) and (4.16); Appendix 3]: Emax -- E -l- ~/,

(4.17)

Emax -- "r/~z~ -l- " r t / ~ .

(4.18)

The differential form of Eq. (4.18) is (Kaelble, 1971) dEmax/d'r = (1/G)(O'r/Ot) + ~/~l.

(4.19)

III. Mathematical Modeling

81

CREEP CREEP

RELAXATION

100 -

75 ~

(%) 63

~

50-

s

25-

's

/i

=

I

II

vl

--trd-- I

I_trd_l

IOSS t

TIME STRESS RELAXATION

100

t~=k

75

1; 50 37 25

---"--

trl "-------I

t

TIME Figure 2 Graphical representation of the Voigt-Kelvin model (a) and the Maxwell model (b) of viscoelasticity, trd is the retardation time and trl is the relaxation time.

82

4. Concentration Regimes and Mathematical Modeling

U n d e r constant T, OT/Ot = 0 and Eq. (4.19) is simplified to (4.20)

dEmax/dt - v/T],

in harmony with the principle of the consistometer that the "thickest" fluids (highest ~li) suffer the least deformation and travel the shortest distance in any given interval. The integral form of Eq. (4.18) (Kaelble, 1971) shows that e is an exponential decay function of t/(T]i/G). The dimensions of T]/G reduce to seconds (Appendix 4) and the equation reaches a limiting 1/e (0.37Ema x) in t = "q/G seconds. The retardation time (trd) is the time required for Emax of a Voigt-Kelvin fluid (Fig. 2a) to be reduced to 37% of Emax after T has been removed (Barnes et al., 1989; Seymour and Carraher, 1981). A long retardation time is characteristic of a more elastic than viscous fluid. In the Kelvin-Voigt test, Ema x i s imposed almost instantly and maintained, while a declining T is measured from its corresponding m a x i m u m (Fig. 2b; Kaelble, 1971): Tma x - - G E 3t-

"qe/t.

(4.21)

From this equation, E "-- [ ( T m a x / T ] )

-

-

(Ge/T])]

t,

(4.22)

and by differentiation (Appendix 5), dE/dt-

( 1 / T ] ) ( T m a x -- G E ) .

(4.23)

The integral form of Eq. (4.23) shows that T is a function of 1 exp(-t/(~q/G)) and t = "q/G seconds is the time required for the fluid to recover 63% (1 - e) of its original shape at t = 0' or, stated differently, to be reduced to 37% of its m a x i m u m deformation at t = 0 (Fig. 2b). The relaxation time (/1; tr 1 in Fig. 2.b) is the time required for T in a Maxwell fluid to be reduced to 37% of its m a x i m u m value at t = 0 (Seymour and Carraher, 1981; Barnes et al., 1989). If log e or e/T VS t is a straight line, the test fluid conforms with the Maxwell model; if the test fluid does not conform, an average t 1 is taken from a spectrum of relaxation times (Mohsenin, 1980). Deformation ( + AE) and recovery ( - A E ) along dissimilar pathways beget hysteresis. The elastic segment of creep and relaxation can occur at the same rate only when there is no hysteresis. Accordingly, in the absence of hysteresis, t I is the time required for a viscoelastic fluid to reach 63% of the m a x i m u m deformation u n d e r stress.

III. Mathematical Modeling

83

m

t1

is related to M w (Elbirli and Shaw, 1978; Ross-Murphy, 1984) as

follows: t 1 =

(4.24)

6( "qi - "rio)M--w/[ (pi)2ciRT ] .

Silberberg (1989) implied the answer to the question why some polysaccharide gels are flow-reversible and others are not: reversible gels have short t 1 and irreversible gels have infinitely long t a m m e a n i n g the coil-stretch transition of a seemingly irreversible gel has an indefinite interval within which to reverse itself after the deforming stimulus has been withdrawn. In oscillatory shear r h e o m e t r y (M. A. Rao, 1992; V. N. M. Rao, 1992), a sinusoidal wave is applied to a dispersion, and the phase amplitudes and differences are measured and related to viscoelasticity. The data yield a complex ~1 (~q*): TI* = G*/c0.

(4.25)

G* is the complex modulus. The shear frequency (0~) oscillates in radians per second between 0 and 90 ~ There are two c o m p o n e n t s to G*, viz., G' (the storage modulus) and G" (the loss modulus), so that ~1" = f ( G ' ) + f(G"). In a dispersion or suspension, G' is a quantitative measure of elasticity and G", of viscosity. W h e n G ' = 0, the fluid is completely viscous and + AE is dissipated exclusively in viscous transport; when G " = 0, + AE is stored in elasticity. G' is a reliable m o n i t o r of the sol-gel transition (Fig. 3).

3000 2000 Pa-sec

9

G'

9

nO

9

G"

9

0

1000 .......

III

10

I

,-=o~,D,,o,,,,,,,,,~, Z~ _ _ . m _ _ _ _ u n ~ ~

i

I

i

i

u

i

20

30

40

50

60

70

,,

80

*r Figure 3 Profile of a complex viscosity(0*) and the storage (G') and loss (G") moduli of a 1% konjac-xanthan-methylcellulose mixture in a 1:1:1 ratio, illustrating the onset of gelation at Tgel approximating 60~

84

4. Concentration Regimes and Mathematical Modeling

In the equation emax = e0 sin tot,

(4.26)

is measured as the amplitude of the strain wave at time t 0. Typically, a fixed and a free surface coated with a sample oscillate in or out of phase in proportion to the degree of viscoelasticity. Recalling that 90 ~ p i / 2 rad and that the sine of an angle in radians added to p i / 2 equals the cosine, the fraction of 9 in phase with e may be represented by G'(to)sintot and the out-of-phase ~, by G"(to)cos tot. Then, at any to, Tmax is the sum of the fractional "r (Barnes et al., 1989): e0

Tmax

=

E0[G t sin tot + G" cos tot ].

(4.27)

J. The Activation Energy of Viscous Flow Equation (3.35) is a precursor to the Arrhenius equation relating the rate constant [k in Eq. (3.30)] to the probability of molecular collisions (to) and the activated energy (E~) of a reaction. Polysaccharide viscous flow is characterized by a modified Arrhenius equation in which ~li/~q0 replaces k:

"qi/~qo

=

CO

exp(+Ea/RT).

(4.28)

E a is now the apparent activation energy of viscous flow. co is called the frequency factor, because it indicates the probability and frequency of the collisions. Severs (1962) made the following observations on dispersions: random polymers change their configuration easily; the most polar polysaccharides require the highest Ea ; E~ is less at higher than at lower T; T has a more drastic effect on polar polymers than on nonpolar polymers. With increasing T, random coils expand and a larger solute surface and hence higher ~qi (greater resistance to flow) should be effected, but for the heightened Brownian activity that tends to lower -%. Decreasing T has the opposite effect. Inasmuch as random polymers are easily disfigured by heat, it may be inferred that amorphous polysaccharides decompose in a temperature range lower than do rods, helices, and crystallites. Polysaccharide deploymerization in the presence of oxygen was found to occur at E~ = 50-105 kJ mo1-1 (11.9-25.1 cal mol-a; Bradley and Mitchell, 1988). The E~ of a cowpea starch gelatinization was reported to be 233.6 kJ mo1-1 (Okechukwu and Rao, 1996a). E~ was substituted for the time-based empirical criterion used in the jelly trade to classify pectins as slow-set, rapid-set, etc. (Walter and Sherman, 1981).

IV. Size

85

IV. Size The size of a polysaccharide normally refers to M or the D P n a restriction that makes no allowance for or, Vex, and vf--all in a dispersion capable of being many times that of the primary structure at or near 0. Hydration, charge, i, and T are indirect determinants of size.

A. The van't Hoff Equation With the discovery that polymer molecules in very dilute solution behave similarly to gas molecules, the osmotic pressure g e n e r a t e d by the former in solution was equated with the gas pressure [Eq. (3.1)]. The concentration d e p e n d e n c e of "rr in dilute solutions is stated as m

(4.29)

'rr = R T ( ci ) / M n .

c i (in grams per liter; c i / V i) is the weight of polymer of molecular weight

Mn. This equation governs m e m b r a n e osmometry of macromolecules, and because its origin is in the combined gas laws, dilution theoretically minimizes errors arising from interparticle interaction. At the same weight concentration, -rr is high for small molecules, but vanishingly low for polymers, due to the latter's low molarity. The quotient 'rr/c i is defined as the reduced osmotic pressure, and if 'rr/c i vs c___/ (usually a straight line) is extrapolated to c i = 0, the intercept is R T M n 1. The equation holds for aqueous dispersions of nonionic polysaccharides and, allowing for D o n n a n distribution, for ionic polysaccharides dispersed in a dilute concentration of electrolytes. Equation (4.29) is the secondary origin of other equations that explain macromolecular solution behavior. For rigorous analysis, Eq. (4.29) is e x p a n d e d to =/c; = n r / M - - . +

+

'cy + ... ).

(4.30)

[3 = 2RTM----~-1 is called the second virial coefficient; it yields the same qualitative information about interaction a s k[~'l]2 in Eq. (4.6). Membrane osmometry seldom requires an accuracy to more than 13'c2 . Doi and Edwards (1986) define a dilute solution as one in which [3 = 0 n t h e ideal condition for accurately measuring M n . T o is that temperature where [3 = 0 (Alberty and Silby, 1992). The fact that [3 provides information about solute-solute interactions, micellization and demicellization studies are made possible by the use of Eqs. (4.29) and (4.30).

86

4. Concentration Regimes and Mathematical Modeling

B. Light Scattering Small, randomly distributed, neutral, noninteracting hydrocolloidal solute undergoes Brownian motion and consequently localized concentration inhomogeneities in a solvent. Oscillating electric fields from a beam of horizontal, plane-polarized, or laser incident light (I 0) transmitted through such an inhomogeneous system induce dipoles in polarizable solute that then scatter light in all directionsmuniformly for spherical particles with diameters less than the wavelength ()t 0) of I 0 (Rayleigh scattering), and nonuniformly for particles with diameters approximating )to (Debye scattering). In analogy to Lambert's law, the reduced intensity of the scattered light (I) is stated as

I / I o = exp( - "rxc).

( 4.31 )

x is the distance traveled by the scattering beam and "r, called the turbidity, is the scattering equivalent of the absorption coefficient in an absorbing solution. I is of the order of 10 -6 times the intensity at I 0 . Turbidimetry is the difference measurement of I in transmitted light 180 ~ from the source of I 0 (unscattered); nephelometry is the difference measured 90 ~ from the source (scattered light). Quantitative nephelometric data are reliable only to the extent that particle sizes and dispersions are reproducible. Equation (4.31) provides a quantitative method of analysis and of determining molecular weights and sizes for spherical particles in the 0.01-2-lxmdiameter range (Beyer, 1959). Some linear hydrocolloids have small enough dimensions ( < 0.1)t) for them to behave as isotropic scatterers (Sperling, 1986). Large, nonspherical particles scatter light anisotropically, causing a number of waves to arrive at the detector in an identical phase, and other waves to arrive in different phases. I is more intense when waves arrive in an identical phase (constructive inference) and less intense when they arrive out of phase (destructive interference). Constructive interference is most pronounced at forward angles (qJ < 90 ~ where the diffracted rays reinforce each other's intensity: destructive interference, especially acute when particle diameters approximate )t (Debye scattering), is most pronounced at backward angles (qJ > 90 ~ where the out-of-phase frequencies have an attenuating effect on each other. It is easy to understand how polydispersity, through the different size effects, can complicate M w determinations by light scattering. The angular dependence of I introduces the laws of sine and cosine into Eq. (4.31):

R , = I , x2/[V~Io(1 + cos 2 , ) 1 .

(4.32)

R , (in cm -1) is called the Rayleigh ratio and I, is the intensity of the scattered beam measured at ~. At 90 ~ cos 2 ~ = 0 and R o = I , x2/ViIo .

IV. Size

87

A dissymmetry method (Z) has been used to characterize polymers by employing a ratio of 145o and Ia~5o (with solvent and depolarization corrections): (4.33)

Z = I45o/Ia~5 o .

For isotropic scatterers at 45 ~ and 135 ~ 1 + COS 2 I]/ has an identical absolute value that cancels to unity. For anisotropic particles, the concentration dependence of Z complicates the measurements, but this defect is mitigated by plotting a series of Z-a vs c~ points and extrapolating to 0. The intercept, referred to as the reduced specific dissymmetry, is a better index of nonsphericity. Z can be adapted to the determination of M w (Stacey, 1956). The application of static light scattering to polymers is based on the theoretical equations of Debye (1944, 1947) and the methodology of Zimm (1948). The principles apply equally to polysaccharides (Sorochan et al., 1971). In total intensity light scattering, monochromatic light (436 and 546 nm) at constant T passes through the dispersion and becomes plane polarized; the horizontal beam is scattered in accordance with the equation (Hiemenz, 1986) (4.34) is in grams per centimeter cubed, n o is the refractive index of the solvent, is the refractive index of the solution or dispersion containing component i, and d n i / d c ~ , the refractive index increment, is the average of the sum of a series of increasing n i - n ~ differences, each difference divided by the respective ci . It is observed from Eq. (4.34) that the scattering intensity is inversely proportional to the fourth power of )t; this is the reason why scattering at the blue end (436 nm) of the electromagnetic spectrum is far more intense than at the green end (546 nm). The first derivative of Eq. (4.30) is ci

n i

O'r

i

= R T ( M ~ -1 + 2f3c i + "" ).

(4.35)

Inasmuch_ as molecular weights obtained from light scattering approximate more closely than M n , the necessary substitutions for O w / O c i from Eq. (4.35) and Mn are made in Eq. (4.34) to give

M w

I~/I o = {2"rr2[n~

+

cos 2 ~/)}/{)t4Nr2(mw I + 2~ci)}.

(4.36)

w is the number of scattering particles per gram of solute. Combining all constants into K (mole times centimeter squared per gram squared),

N/M

88

4. Concentration Regimes and Mathematical Modeling

Eq. 4.36 is rewritten R , = K c i / ( m ~ o 1 "}- 2 f 3 c i ) ,

(4.37)

K c i / R , = M---~ 1 q- 2 ~ c i .

(4.38)

Equation (4.38) is a straight line with intercept M-~ 1 and slope 213 (Hiemenz, 1986). Destructive interference is compensated for by inserting a size- and shape-dependent particle factor [II(~)] in Eq. (4.38): Kc;/R,

= [ 1 / H ( , ) ] [J~w 1 + 213q].

(4.39)

For the smallest scattering particles and in the direction of I 0 (transmitted beam) for the largest, I I ( ~ ) = 1. For Rayleigh scatterers (Hiemenz, 1986), .

--

ci ]//[3N)K4 (d w 1 -}- 2

and formally combining the constants into H c i / . r = M--w-1 + 2 ~ c i .

H,

ci)] .

(4.40)

Eq. (4.40) is simplified: (4.41)

When I I ( ~ ) = 1, Eqs. (4.39) and (4.41) are indistinguishable. For the most common conformations (random-coil, rod, and sphere), II(O) is a function of s i n ~ (Stacey, 1956; Allcock and Lampe, 1981; Hiemenz, 1986; Cowie, 1993). K c i / R , vs s i n 2 ( ~ / 2 ) + kc i is a Zimm plot utilizing Eq. (4.39). Zimm plots are applicable to polyanions in their most probable conformation, but with negative slopes (Berth and Lexow, 1991), because of their propensity to ordering effected by charge repulsion (Stacey, 1956). Light-scattering measurements of polyelectrolytes in salt-free solutions and dispersions have borderline accuracy (Dautzenberg et al., 1994). The adaptation of Z [Eq. (4.33)] to M w determinations presupposes knowledge of conformation. A possibly false assumption of geometry is avoided by deriving M w from a Zimm plot in which extrapolation to ~ = 0 ~ eliminates optical interference and extrapolation to c i = 0 eliminates solute-solute interactions. The two lines converge at the intercept containing Mw 1, with the slope of the c i = 0 line directly related to R g2 and the slope of the ~ = 0 ~ line directly related to 213. The same quantitative and qualitative information about interaction is deducible from osmometry and light-scattering 13 (Frank and Mark, 1955). A light-scattering M w computes to a higher value than an osmometry M____,, because the statistical origins specify M w = f ( Y _ , s i w i 2 / ~ s i w i ) , whereas M n = f ( ~ s i w i / Y _ , s i ) , where s i is the number of particles of i in a weight category and each category is of average molecular weight wi .

IV. Size

89

Dynamic light scattering (Everett, 1988; Sun, 1994) is a comprehensive description of certain optical p h e n o m e n a that are adapted to the measurem e n t by an autocorrelator of short-life fluctuations in I 0 and I, in the k range 488-635 nm, at different constant angles (0 ~ to ~), over intervals of 10 -7 s. The m e t h o d exploits the high resolution of laser beam frequencies (co). At a constant temperature, the fluctuations result in a spectrum of co observed as twinkling by the unaided eye, as the particle scatterers change vibration, rotation, translation, and hence localized densities. The product I o . I , is an important parameter that depends on the time scale of the measurements. At any ~, I,
(4.42)

( 4 p i n o X o ) / ( s i n k / 2 ) is termed the scattering wave vector (O). I o . I , , known as the autocorrelation or time correlation function [abbreviated t o g(t)], decays exponentially:

g ( t ) =Io2 e x p ( - t / t ~ ) .

(4.43)

For monomolecular spherical models, tc = 1/(DO2).

(4.44)

t c is denoted the correlation time, the relaxation time, or the decay time; - t / t ~ is the slope of a computer plot of lng(t) vs t. The hydrodynamic radius R h is related to t c by the equation (Everett, 1988) R h = (kT/6pi'qo)OZtc.

(4.45)

Experimentally, t c is extracted from a series of ln g(t) at ~, plotted against 0 2 to give a straight line. Deviations arise from size polydispersity, because a different R h of each fraction yields a different t c. A graph of t c vs 1 / ~ 2 yields Rg. For spherical particles, R h = Rg and for equivalent hydrodynamic spheres, R h = 0.665Rg (Tanford, 1961). A statistical analysis of light-scattering data can compensate for polydispersity. In cumulant analysis, ln g(t) is e x p a n d e d in a power series and coefficients of the different terms are evaluated against the experimentally obtained t, in search of the closest-fitting average selected by the smallness of the standard deviation. In a histogram method, the experimental t is

90

4. Concentration Regimes and Mathematical Modeling

evaluated against t of a known and variable composition of a small population of discrete particles and the best fit chosen. On the basis of light-scattering experiments, Burchard (1994) concluded that molecularly dispersed galactomannans could not be prepared, nor was the use of hydrogen-bond breakers satisfactory in accomplishing the dispersion. In his estimation, galactomannan dispersions do not reach a state of thermodynamic equilibrium. By laser diffractometry, Okechukwu and Rao (1996a) found that the ratio of the major and minor axes of ellipsoidal, ungelatinized cowpea starch granules was in the range 1-1.8 ~m.

C. The Contour and Persistence Lengths In the equivalent sphere model, the volume v i = 4 p i R ~ / 3 . For spheres, Rg .~ M 1 / 3 ; for random coils, Rg ~ M 1 / 2 ; for rods, Rg ~ M (Ross-Murphy, 1994). Specifically from the Zimm plot (Cowie, 1991), Rg2 = 3kz[3~w/16(pi)2 .

(4.46)

The ratio of Rg of a linear and a nonlinear polysaccharide with the same composition and DP is a possible index of branching (Zimm and Stockmayer, 1949). If two lines (/1 and 12) are imagined to be the distance of each of the two ends of a primary chain from the center of mass (or from any point on the primary structure), a third line ~ between the two ends forms a triangle with 11 and /2: ~ obeys the law of cosines. 15 The random flight model of an unsubstituted linear polymer chain in which there are no steric or energetic hindrances assumes equal bond lengths and all possible bond angles between atoms or segments. This freely jointed chain has a distance r ~ (the root mean square end-to-end distance) between its two ends that is theoretically calculable from the law of cosines of the bond angles. All configurations of the primary structure are equally probable at equilibrium, with an average of tightly coiled chains in a Gaussian distribution of r ~ (Eisenberg and King, 1977). The effect of a good and a poor solvent on curling and uncurling of a linear molecule (Alfrey et al., 1942; Severs, 1962) is on r ~ (Banks and Greenwood, 1975). The contour length (Flory, 1953) is the fully extended length of a linear polymer, readily visualized in an unsubstituted polyanion reacting to electrostatic repulsion. The real l e n g t h m t h e persistence lengthmis fixed by substitutions, branching, kinking, interactions, etc., that cause the chain to "persist" with a dimension less than the contour length. The contour and

ll.l 2

15. Defining ~2 in terms of the sum of the squares of 11 and 12 minus twice the product of and the cosine of the angle made by the lines intersecting at the center of mass.

IV. Size

91

persistence lengths are conceptually equal for a linear, unsubstituted polyanion. The contour length of a polyanion may be more than twice that of the neutral molecule's length with an identical DP (Veis and Eggenberger, 1954).

D. The M a r k - Houwink Equation An equation finding much application to polysaccharides is the Mark-Houwink equation" m

[~1] = k M ~ .

(4.47)

k and v are systemic to each polymer solution and are known to be variable with T, solvent (Alfrey et al., 1942), i, fine structure (Fasihuddin et al., 1988), chain stiffness (Morris et al., 1981), branching (Bahary, 1973), and heterogeneities. There is evidence that v itself is influenced by M (Deckers et al., 1986): Carpenter and Westerman (1975) give this dependence as the reason why Eq. (4.47) is not valid over the entire range of M. The determination of M with Eq. (4.47) requires prior knowledge of k and v, that in turn require a homologous series of standards for quantification. Unlike synthetic polymers, the biocolloidal M cannot be known with certainty using this equation, because there are no homologous standards with which to predetermine k and v. For this reason, Eq. (4.47) is referenced against absolute methods (membrane osmometry and light-scattering photometry); however, the equation is an economical alternative to expensive an__alytical instrumentation. M so obtained is an ~l-average molecular weight (My). Table II lists a number of Mark-Houwink constants reported for some polysaccharides. Constants for other polysaccharides were tabulated by Launay et al. (1986) and Lapasin and Pricl (1995). v, usually varying between 0.5 and 1.0, may serve as an index of chain conformation. A value of 0.5 is estimated for a random coil in a 0 (non-freedraining) solvent (Daniels et al., 1970; Cowie, 1991; Hiemenz, 1986) and 0.8 is estimated in a good solvent (Cowie, 1991). Mitchell's (1979) range is i

TABLE II Mark-Houwink Constants for Some Dispersed Polysaccharides

Polysaccharide Amylose Konjac Cellulose Guar

Solvent

k (cmS/g)

v

Reference

0.68 0.74 0.9-1.19 0.723

Burchard (1963) Kishida et al. (1978) Stannett(1989) Morris (1990)

H20

1.32 X 10 -2

H20

6.37 X 10 -4

H20

3.8 X 10 -2

92

4. Concentration Regimes and Mathematical Modeling

0.5-0.8 for a random coil by the equivalent sphere theory, 1.0-1.2 for a free-draining random coil, and 1.8 for a rod. For a free-draining coil, v = 1 (Daniels et al., 1970; Hiemenz, 1986). Chain compaction is refleced in v = 0.1-0.3 (Robinson et al., 1982). K is related to the macromolecular geometry, large for expanded and rigid coils, e.g., cellulose, pectin, and alginate (Lapasin and Pricl, 1995).

E. The Hydrodynamic Volume By definition, M['q], canceling to centimeters cubed per mole, is the hydrodynamic volume (Barth, 1986). M. d-1 (Glasstone and Lewis, 1960) has identical dimensions: [xl]M = M---/di-1 .

(4.48)

di -1 (reciprocal density) is defined as the specific volume. [TI]M is adapted to

advantage in gel chromatography (synonymous with size exclusion chromatography) whereby size-homogeneous fractions of M elute from the same column, each in a unique elution volume (Vel), regardless of conformation. Every chromatographic column has a molecular cutoff value or fractionation limit, and molecules whose sizes are above this limit elute in a void volume (v 0) without the equilibrium-disequilibrium-equilibrium times necessary for baseline resolutions: these sizes are "excluded" from the gel micropores. Within the fractionation range, noninteracting molecules theor__etically elute in order of decreasing hydrodynamic size; i.e., the larger the M, the smaller is the v d , because the larger fractions are resolved in the earlier stages of elution. Smaller M elute in larger V~l and identical sizes elute in an identical v d . A rod elutes sooner than a random coil for macromolecules of equal M (Rollings et al., 1983). A gel bed possessing a negative charge hastens the elution of polyanions, because of electrostatic repulsion and also because the polyanions are effectively barred from entering the gel micropores. Polysaccharide gel-be__d surfaces have a fractionally negative charge. If log[TI]M for each eluted fraction of a standard polysaccharide is plotte__d against its corresponding Vel, a universal graph circumscribed by log {[ "q]M }standard VS {/)el}standard may be drawn that fits unknown polysaccharide fractions with identical vr An unknown M may be computed by substitution in the equation

{[TI]m}standard = {[~]m}unknown"

(4.49)

The procedure is as follows: a standard graph of {[TI]M}standard VS /)el is constructed, [~q]unknown is measured, and {[~l]M }standard corresponding to the unknown Vel is read from the graph. Inserting the data in Eq. (4.49),

93

IV. Size m

(M)unknown is then calculated. The method is accurate for linear chains, but fails for branched and probably stiff polymers (Burchard, 1994). By another definition, the hydrodynamic volume is the volume of a single particle in solution (Tanford, 1961); or by invoking the unsolvated, spherical molecular model, ( 4 / 3 ) p i ( r ~ ) s cm 3. The total volume of particles in 1 g of a dispersed solute is ( 4 / 3 ) p i ( r ~ ) ~ cm3(N/M). According to Seymour and Carraher (1981), ( r ~ ) 3 cm 3 is the effective hydrodynamic volume. In terms of the specific volume, d~-1 (cmS/g) = ( 4 / 3 ) p i ( r~)SN./~/- a(cmS/g).

(4.50)

[xI] has the same unit as d-a and it follows that 1

(4.51)

is the Flory viscosity constant. For chains (Cowie, 1991), r ~ = ~/(6Rg) 2 .

(4.52)

Short, stiff polymers behave like rods (small r) whose model as a wormlike chain has r ~ the same as the contour length and Rg much smaller (Elias, 1979). Rg and ~2 decrease with curling, compaction, and age. r~

is related to C

ot2 = fz/f02.

through ot (Sun, 1994): (4.53)

Using C as the unperturbed chain reference and allowing for expansion and contraction, = |

-'

(4.54)

At 0, e~ = 1. Making the necessary substitutions for C o 2 and ot in Eq. (4.54),

[qq]0-" ~I963/2R0S]~/-1. F"X-.

(4.55)

The Rg-1/~"~ relationships permit inferences to be made about macromolecular conformation, i.e., whether or not the molecule is flexible, linear, rodlike, branched, etc. Equations (4.6)-(4.11) differ from Eqs. (4.50)-(4.55) in not having the radius of a sphere as a variable, but having concentration replace size.

94

4. Concentration Regimes and Mathematical Modeling

[B]0 of a nonlinear polymer and of its linear counterpart with identical M have been rationalized into a practical index of the degree of branching (Bahary, 1973). The non-free-draining water attached to a polysaccharide molecule and the free-draining water surrounding it travel at different velocities across a boundary whose location is a function of fc and, therefore, a function of Rg and ~1o [Eq. (4.1); Flory, 1953; Tanford, 1961]:

yc/ o f< = kB[f(Rg)].

(4.57)

F. Fractal Dimensionality Any volume measurement (V) is definable by the cube of a linear dimension:

V = r 3.

(4.58)

Three-dimensional objects occupy space that may similarly be characterized by an equivalency of length times breadth times width. Fractals are such objects: they are irregularly shaped and built upon a constant repeating, microscopic fine structure. A polysaccharide gel, for example, is generated from an almost infinite number of scale-invariant nuclei (Birdi, 1993) multiplied many times into tertiary and quaternary structures. Assuming Rg to be the constant dimension of the fractal nucleus, flocs conform to

V=RgD .

(4.59)

D is the fractal dimensionality. If Rg is divided into r sublengths, Eq. (4.59) becomes

V = ( R g / r ) D.

(4.60)

Letting Vt be the total volume of a gel,

Vt = ( R g / r ) 3

(4.61)

and at less than capacity filling,

V/li t = (Rg/r)D/(Rg/r) = ( R g / r ) I)-3.

~

(4.62) (4.63)

V / l i t of a gel has a meaning resemblingthat of +i. The determination of the volume fraction of solute in a sol (+i) is a simple computation from a series

IV. Size

95

of density-concentration measurements (Walter and Matias, 1989). The incidence of junction zones is proportional to +i. A random-coil polymer chain in solution has D between 1 and 2 (Birdi, 1993). It is the nature of flocculation and aging to increase the size of the dispersed hydrocolloidal units at the expense of their number. This suggests that it is possible to have a dispersion in which the original, discrete molecules having a low c i or 4)i become larger and highly overlappingma semidilute solution (Doi and Edwards, 1986). When there is a gelation phase change, +i approximates 1 as the dispersion approaches the condition of a xerogel. Considering Eq. (4.55), given the magnifying influence of solutesolute interaction on ["q]i and since M i is constant, it can be concluded that R g and R h in a sol or gel is larger than R 0 and that neither a hydrophobic nor 0 environment accomplishes Rg. Walstra et al. (1991) discussed some improbabilities of the fractal theory of gelation centering mostly on the oversimplification of the fractal model that culminates in Eq. (4.63). Surface area and porosity are examples of polysaccharide properties other than gelation that are amenable to fractal analysis. G. Sedimentation

By accelerating deposition with a slowly rotating ultracentrifuge, Stokes' law [Eq. (4.1)] is modified in uniform circular motion to an equilibrium distribution along the axis of rotation (0 to x), as a function of solute mass (mi). At higher centrifugal velocities, sedimentation succeeds the equilibrium distribution. Sedimentation equilibrium and sedimentation velocity provide a means to determine M. The fundamental origin of the relationship between centrifugal force ( F ) , m i , angular velocity (~It), and distance from the centrifugal axis (x') is F--mi~itZ/x' (Smith and Cooper, 1957). From this relationship, equations were derived to describe the translational vector during distribution and deposition in a centrifugal field from 0 (the meniscus) to x (the bottom of the cell) along the rotational axis (Williams et al., 1978). 1. S e d i m e n t a t i o n E q u i l i b r i u m

F changes along x according to F = mi~It Z x .

If

(4.64)

m i = M w,

F=Mi~2x.

(4.65)

96

4. Concentration Regimes and Mathematical Modeling

Ordinarily a dispersed m i can be represented as the product of density and partial specific volume (div i) and v i, having dimensions of area times length ( A x ) , any infinitesimal change in A x (OA Ox) is accompanied by an infinitesimal change in F (OF). Substituting in Eq. (4.64) and rearranging, (4.66)

O ( F / A ) = digit 2 x OX.

Recalling the definition of pressure ( F / A = ~r) and substituting for F / A Eq. (4.66),

in

(4.67)

O'rr/Ox = d i l l t2 x.

Simultaneously at every Ox, there is an infinitesimal change in concentration (Oc i) and consequently an infinitesimal change in the chemical potential (0Ix) and the enthalpy change [0(AHmix)]. From the energy relationships at OX, 011, ,~ 0(AGmix). The equation AGmi x = A H m i x - T ASmi x [Eq. (3.18)] is differentiable to O( ACmix)/O

=

= Vm .

(4.68)

m

Given

lI m

=

viMw,

Olx/O'rr = v i M w .

(4.69)

At equilibrium, solute transport through the solvent ceases, two gradients affecting 0 ( A H m i x) and counterbalancing F (viz. Oc/Ox and O'rr/Ox) are established, and the centripetal and centrifugal forces are equal: MwXit 2 x = ( Oc i / Ox ) Ob~/ Oc i + ( O'rr/ Ox ) OIx/ O'rr .

(4.70)

One form of Raoult's law [Eq. (2.5)] differentiates to Oix/ Oci = R T / c i .

(4.71)

An ultracentrifuge is designed to facilitate readings of ci at different lengths along the rotating axis (x 0, x I , x 2 , x 3 . . . . . x); so Oci/Ox in Eq. (4.70) is experimentally determinable directly. Substitution in Eq. (4.70) of the partial derivatives from Eqs. (4.67), (4.69), and (4.71) and the experimentally determined Oci/Ox gives M i R t 2 x = ( O c i / O x ) R T / c i + diaIt 2 x.viM-- w .

(4.72)

Rearranging, M--w~Ir2x =viM-- w diXIt2 x + ( R T / c i ) ( O c i / O x ) ,

(4.73)

[ 1 / ( x Ox)] Oci/c i = xIt 2Mw(1 - v i d i ) / R T ,

(4.74)

IV. Size

97

( 1 / x Ox) integrates to 2//x 2, and and transposition, ( 2 / x 2 ) l n ( c i / C o ) = ~It2Mw(1

-

ln(Cl/Co) = .22~w(1 - v i d i ) ( x

(OCi/Ci) integrates

to ln ci. By substitution

vidi)/RZ , 2 -x2)/2RT.

(4.75) (4.76)

s and co are the steady-state concentrations of i at Xl, and x 0 and ( 1 - r i d i) is called the buoyancy factor: if 1 - vid i is positive, there is deposition; if negative, there is flotation (Cowie, 1991). v i and d i are independently measurable. A graph o f l n ( c i / c o) vs (x 2 - x 2) has slope ~ 2Mw(1 - v i d i ) / Z R T from which M w may be extracted. 2. Sedimentation Velocity

In the event of sedimentation, x is the changing solute boundary (the meniscus) distance away from the meniscus at t o, the initial position of the meniscus. If the weight of one molecule M w / N is substituted for m i , Eq. (4.65) is F = ( M w / N ) ~ 2 x.

(4.77)

For sedimentation to occur, it is necessary that F exceeds buoyancy only slightly in proportion to vid i (Archimedes principle) and the frictional resistance (f~ Ox/Ot) [Eq. (4.2)]:

( Mw/N) ILIf2x = ( Mw/N) ~If 2X divi q-L ax/Ot,

(4.78)

M w / N x l r 2 x - M w / N x l r 2x divi = fc Ox/Ot ,

(4.79)

[ 1 / ( xI~2x)] Ox/Ot =Mw(1 - d i v i ) / N f c .

(4.80)

By definition, the sedimentation constant (S~) is Sv=[1/(~2x)]Ox/Ot,

(4.81)

and a plot of log x vs t is ideally a straight line yielding S v . The unit of S v is the Svedberg (10-as s; Hiemenz, 1986). Substituting Sv in Eq. (4.80), Sv =mw(] -divi)/(Nfc fc = ( R T / D N )

(4.82)

).

[Eq. (3.27)], and Eq. (4.82) may be rewritten

Sv =Mw(1 - d i v i ) D / ( R T

) 9

(4.83)

98

4. Concentration Regimes and Mathematical Modeling

A rearranged Eq. (4.83) takes the form of t h e Svedberg equation for determining M w by sedimentation velocity: M w = RTSv/[D(1

(4.84)

- vidi) ] .

Equation (4.84) is absolute, but Sv and D must be known. The concentration dependence of D and Sv necessitates substitution of So from Eq. (4.85) for S. in Eq. (4.84)(Dautzenberg et al., 1994): 1 / S v = 1/S0(1 + k c i ) .

(4.85)

A plot of the series of experimental S~-1 vs c i provides slope So 1. Equation (4.86) (Cowie, 1991; Dautzenberg et al., 1994) is an alternative method of determining an unknown M w that was developed from polymer fractions of known M w : S,

(4.86)

ksM w .

D and Sv are related to the van't Hoff equation [Eq. (4.30)]. For polyelectrolytes, (D/Sv)(1

-vedo)

=RT((1/JMn)

+ ~c + ~'c 2 + "'" ).

(4.87)

"t)e is the partial specific volume of the polyelectrolyte (Dautzenberg et al.,

1994). It is thus possible to procure the second virial coefficient from sedimentation data. The particle radius (r) of homogeneous suspended solute may be calculated with the use of Eq. (4.88) (Nichols and Bailey, 1949): r = [9TIi ln( X l / X o ) ] / 2 o j 2 ( d i -

do) At] 1/2.

(4.88)

At is the time elapsing for the solute to travel from x 0 to x 1 . Equation (4.88) suggests that the most densely flocculated gel (largest d i) has the smallest fractal nucleus. Sedimentation velocity permits the widest possible range of M w m e a surements, from 300 to 108 Da for dispersions and as high as 1014 Da for suspended solute (Dautzenberg et al., 1994).

H. Surface Area The importance of surface area in colloidal chemistry has spurred m a n y attempts to develop a method of its accurate measurement from physical adsorption processes. All of the methods so far are empirical and attended with difficulty involving surface nonuniformity, polymolecularity, conformadonal shifts, and multilayer adsorption. Polysaccharide surfaces are seldom

99

IV. Size

solid and uniform, but are more likely to be three-dimensional and contain channels and pores. Most adsorption data involving physical forces fit the Langmuir equation, stated as (Adamson, 1990; Baianu, 1992a)

Asp - n a a

(4.89)

x .

Asp is the specific surface area of the adsorbent, a x is the cross-sectional area of an adsorbate molecule, and n a is the n u m b e r of adsorption sites, identical to the total n u m b e r of molecules adsorbed at n a a ~ without Vex effects, supposing a uniform monolayer thickness of solute at saturation. At equilibrium, by analogy with an ideal gas, the rates of desorption and adsorption are equal. Before equilibrium, the rate of adsorption of compon e n t i is proportional to c i and the n u m b e r of unfilled sites; the rate of desorption is proportional only to the n u m b e r of filled sites. Letting c i be the solution concentration at any time, cn be the surface saturation concentration, c u be the unfilled-sites concentration, ( c , - c u ) be the filled-sites concentration, k a be the adsorption equilibrium constant, and k a be the desorption equilibrium constant, (4.90)

k a c i C u = k d ( Cn - - C u ) , 1/C u -- 1/C n -+-ka/(kdCn)C

(4,91)

i .

c u , and ci are experimentally determinable quantities. Ideally in dilute solution, Eq. (4.91) is linear, giving slope k a / ( k e c , ) . Equation (4.91) is valid in the vicinity of T c (Kipling, 1965). When all adsorption sites are filled, cn is constant and the isotherm afterward remains at a plateau concentration. If a calibrating substance, e.g., a fatty acid, saturates Asp , the total n u m b e r of adsorption sites is

c,,

(4.92)

n a = dvN/m.

d, m , and v are the density, molecular weight, and molecular volume, respectively, of the fatty acid: dv cancels to grams, cn is the equivalent of the fatty acid dv. Substituting for c n in Eq. (4.89), the calibrating equation is Asp

(4.93)

=aocnN/m.

The fatty acid cross-sectional area (a 0) has been measured at 20.5 ~2 (Adamson, 1990). 16 Fatty acids are suitable standards because they order themselves closely packed perpendicularly to a hydrophilic surface. 16. One angstrom equals 10-10 m or 10- 6

cm

or 10-1 nm.

I O0

4. Concentration Regimes and Mathematical Modeling

V. Summary The size of a topologically linear, random-coil polysaccharide can be determined by adopting the model of an equivalent hydrodynamic sphere, recognizing limitations due to friction, hydration, branching, etc. Dilution mitigates but does not altogether eliminate these effects. Dilute regimes have imprecise boundaries, defined more by practical utility than by rigid physical and numerical criteria. Polysaccharide molecular weights are determinable by a number of methods that relate directly and indirectly to intrinsic (e.g., primary-chain length) and extrinsic (e.g., electrolyte concentration) factors. Light scattering and osmometry, particularly, can be adapted to quantify absolutely the hydrocolloidal mass and its dimensions. Viscometry is not absolute, but because it is simpler and less rigorous than light scattering and osmometry methods, it has become routine in polysaccharide physical chemistry research. A major shortcoming of the application of viscometry to biocolloids, generally, is the lack of primary standards for determining necessary empirical constants. A solid polysaccharide surface is measurable by adsorption in a monolayer of a standard compound like a fatty acid. Nonequilibrium accumulation of adsorbate on the polysaccharide solid surface is a function of time. Ill-defined polysaccharide systems are measurable only as fractal aggregates on whose nuclear surfaces multilayer adsorption of solute can occur.

CHAPTER 5

Additivity, Complementarity, and Synergism I. I n t r o d u c t i o n Mixed dispersions react by the same number-, weight-, and viscosity-average principles as single-solute dispersions, but combinations may evoke ~entirely new oral sensations: for example, combinations of starch and carrageenan were evaluated for texture in cream desserts and were found to elicit different responses (Nadison, 1990); Lopes da Silva et al. (1993) blended high- and low-methoxyl pectins with locust bean gum in different proportions and also found different responses. Pectin is believed to contribute flavor-release characteristics in yogurts that are superior to those of starch (Hoefler, 1991). Between the extremes of complete phase miscibility and separation, the contribution of each component in a dispersion to the different sensations may be additive, complementary, or synergistic with a variety of compounds. Polysaccharides display mutual antagonism under certain dispersion conditions.

II. I n t e r a c t i o n s When two polysaccharides are dispersed in water under gelling conditions, the one more prone to gelation may develop a continuous reticulum throughout the solvent, embedding discrete volumes of the cosolute dispersion. The developed reticulum acquires the properties of the continuous phase polysaccharide and is said to be filled with the cosolute, Alternatively, the ,solute more prone to gelation may form deposition nuclei for the cosolute (Walter et al., 1978). If the cosolutes are equally capable of gelling, two independent reticula may interpenetrate, giving the gel the appearance of a macroscopically homogeneous dispersion. Random-coil polysaccharides

I01

102

s. Additivity, Complementarity, and Synergism

are more likely to interpenetrate than rodlike polysaccharides. What Morris (1992) described as a coupled network may also be possible if segments of each polysaccharide are fused into junction zones. Dispersed in water, polysaccharides that have the same net charge may separate into two liquid phases (coacervates), each containing water, a major concentration of one solute and a minor concentration of the other, and vice versa. Weak synthetic polyelectrolytes develop gelatinous coacervates and strong synthetic polyelectrolytes develop colloidal precipitates (Eisenberg and King, 1977). Depending on concentrations, the two phases may combine into a solid suspension wherein crystallike bodies formed may act as a stabilizer (Horton and Donald, 1991). These mixed solid dispersions have textural inhomogeneities that are in fact points of weakness, possibly subject to rupture under small stress. Rupture sites in mixed gels affect their mechanical strength and consequently their handling properties, utility, and eating quality (van Fliet et al., 1991).

A. Polysaccharide- Polysaccharide 1. Additivity Additivity obtains when a measured property, e.g., viscosity, is the sum of the contributions made by each cosolute. Additivity for any pair of compatible polysaccharides fits the following equation developed for cellulose gums (Hercules, Inc., 1980): log ~qB = [ X log ~q, + (100 - X)log ~12] / 100.

(5.1)

~qB, ~11, and T]2 refer to the viscosity of the blend, the first solute, and the second solute, respectively, and X is the weight percentage of one solute. ~qB may also be estimated graphically by reading ~qa and T~2 from the respective vertical axes of Fig. 1 and connecting them with a first line: from the desired ~]B on one axis, a second line horizontal to the materials-scale axis is drawn to intersect the first line; from the point of intersection, a third line is drawn parallel to the ~1 axes to intersect the materials-scale axis where the relative percentages of solutes 1 and 2 are read at the desired composition. It is doubtful that blends of more than two polysaccharides have merit, considering the wide spectrum of properties of each. Blending may also be accomplished by use of the equation (Dow Chemical Co., 1990) = x,.I/8

X1

and

+

X 2 are

(5.2) the weight fractions of solutes 1 and 2.

II. I n t e r a c t i o n s

10 3

SOLUTION VISCOSITY (cps. AT 25~ I0.000, 9,000 8,000 7,000

-

6,000

-

5,000 -

4.000 3,000

-

2,000

-

1,000 '~ 900 !800

-

500

i-

700 '600 400 '300

/

VlSCl )SITY O : AV~ ~ILABLE POI YMER A

~

'

J

200

~,.,~

10090SO70-

~

VISCOSITY OF_ AVAIL ll,B LE POLY! |ER B

~

DESIREr VISCOS TY I N EXA|~PLE, p.:15

I

f

.

5o-

|I

iI

40-

I

60 -

3o-

I0100

" PERCENT ~ POLYMER B

l I ~

.

.

.

.

. . . . . . . . . . . . 90 80 70

I 1 I I I ! II PC)LYMER |LEND II , j l ~ , ~" NEEDI D I i I I 9 !. . . . . . . . 60 50 40 30

1'0

410

20-

PERCENT POLYMER A

~" ~

w

210

3=0

5'0

10

;~0

.

.

.

. . 20 dO

.

.

-

10

0

9~0

106

Figure I Polysaccharide viscosity blending chart. Reprinted with the permission of Hercules, Inc., Wilmington, DE.

104

5. Additivity, Complementarity, and Synergism

2. Complementarity

One solute may bring to the dispersion an essential property that the cosolute lacks or is deficient in: for example, starch dispersions tend to become undesirably thin by heatingma quality defect in broths and gravies correctible by adding 1-2.9% of thermosetting methylcellulose (Henderson, 1988). The very hygroscopic guar gum is added to starch gels to prevent syneresis after thawing: waxy corn starch was added to curdlan for the same reason (Nakao et al., 1991). Gellan gum shortened the setting time of starch confections from 45 to 24 h (Dziezak, 1991).

3. Synergism The combined properties of the cosolutes in a single phase may exceed that of the additive property of each solute in separate phases in an identical volume of water: this is synergism. The galactomannans, especially those with the least galactose substituents on the primary structure, display synergism with the broadest array of polysaccharides (Dea, 1989; Sprenger, 1990). The unsubstituted mannan region was implicated in the crosslinking of xanthan and locust bean gums with other gums (Dea and Morris, 1977). Helices and rigid rods increase in viscosity upon the addition of a random-coil polymer (Laurent et al., 1974; Lee and Lee, 1979; Reinhart, 1980). K-Carrageenan is synergistic with locust bean gum, but not with guar g u m (Moirano, 1977), and with konjac flour containing 85% glucomannan in an optimum range of 30:70-50:50 to a total concentration of 0.6% (FMC, 1989). Some characteristics of the blend are a property of the konjac molecular weight (Kohyama et al., 1993). Carrageenans, pregelatinized starch, and pyrophosphate make a formula for cold-setting milk puddings (Guiseley et al., 1980). Blends of starch and konjac gum (Tye, 1991) and of some modified celluloses (Hercules, Inc., 1980) are synergistic. A ternary dispersion of 1% methylcellulose and 2.9% starch had almost 2.5 times the viscosity of a blend of 0.5% methylcellulose and 2.9% starch, and approximately 20 times the viscosity of 3.9% starch (Hegenbart, 1989). Methylcellulose-starch viscosity synergism was suggested as a formula to decrease caloric content (Henderson, 1989). Guar gum can increase starch paste viscosity tenfold (Christianson et al., 1981). Xanthan gum has a remarkable affinity for 1,4-[3-D-linked polysaccharides with which it forms thermoreversible (Williams et al., 1991) viscoelastic (Tye, 1991) gels. Deacetylated xanthan is especially synergistic (Tako and Nakamura, 1984; Lopes et al., 1992). Neither xanthan nor locust bean gum by itself is a gelling polysaccharide, but together, there is an approximately hundredfold viscosity increase (Kelco, 1976), ultimately terminating in firm, rubbery gels (Dea and Morris, 1977) at equal concentrations totaling 0.1% (Rocks, 1971). Maximum gel strengths of these blends are in the quotient

II. Interactions

105

range of 3 / 2 - 2 / 3 (Kovacs, 1973; Clark, 1988). Xanthan gum does not gel with guar gum, but shows viscosity synergism with it. Xanthan-konjac blends gel synergistically and thermoreversibly (Williams et al., 1991). Xanthan-locust bean gum gelation is a lock-and-key mechanism that guar gum does not fit (Rocks, 1971). Brownsey et al. (1988) came upon evidence of the denaturation (disorder-order transition) of the xanthan helix, and of binding of the stereochemically compatible cellulose chains. Having studied a series of stereochemically compatible synergistic and nonsynergistic blends of xanthan and other gums, Morris (1992) concluded that denaturation of the xanthan helix was necessary. High-methoxyl pectin and alginate, nongelling polysaccharides by themselves, solidify into cold gels without sucrose when the mixture is acidified (Morris and Chilvers, 1984; Toft et al., 1986), although 65% sucrose would otherwise be required to gel the pectin. Low-methoxyl-pectin gels are thermoreversible, but some mixtures with propyleneglycol alginate mixture may be irreversible (Toft, 1982). A study of alginate-pectin mixtures reinforced the belief that regularly spaced atomic groups, arranged in parallel chains, lead to strong, cooperative bonding. Alginate contains blocks of mannurate and ot-L-guluronate; pectin is an OL-D-galacturonan that is mostly a mirror image of ot-L-guluronic acid: the two are synergistic below pH 3.8, upon slowly acidifying the mixture. Presumably, paired segments of galacturonic and guluronic acids containing at least four deesterified monomers each develop strong, cooperative, interchain contacts (Toft et al., 1986). It had been thought that the molecular basis of synergism was bonding in mixed junction zones, developed as a result of cooperative association (Blanshard and Mitchell, 1979), but current evidence points also to polymer exclusions (Morris, 1990; Doublier and Llamas, 1991) and lock-and-key interactions (Rocks, 1971). Mutual exclusion as well as aggregation of mixed helices (Kelco, 1976) possibly explain the viscosity synergism in dispersion blends of galactomannans and helical polysaccharides. Xanthan with the galactomannans, and xanthan, guar gum, and CMC with starch (Christianson et al., 1981), showed this kind of synergism. Exact mechanisms may not be completely elucidated, but there is consensus that dissimilarities in localized chemical structures play an important role (Dea, 1989), arguing unavoidable entrapment of one polysaccharide in the gel of the other. Turquois et al. (1992) appeared to support the entrapment hypothesis by suggesting that synergistic gelation between K-carrageenan and locust bean gum results in network coupling.

B. Polysaccharide- Lipid Lipid matter in fruits and vegetables, omitting the wax coating, is low enough to be considered insignificant. However, many polysaccharide iso-

106

5. Additivity, Complementarity, and Synergism

lates contain traces of lipid matter that are occasionally isolated with them, sometimes in large quantifies from oleaginous tissues.

1. The Nature and Properties of Lipids Naturally occurring food lipids are composed of short- and long-chain fatty alcohols, acids, esters, and conjugates: glycerol is the dominant alcohol. Almost invariably, they consist of straight-chain fatty acids, which help to predict their physical state on the basis of incremental additions of mass 14 (-CH2-). Shorter chains are more likely to be liquid (oil) and longer chains are more likely to be solid (fat). Physical properties also depend on the degree of fatty acid unsaturation and on the relative fatty acid positions on glycerol. Animal lipids tend mostly to be saturated fats and plant lipids tend to be unsaturated oils. Fatty acids containing up to 10 carbons (capric acid) are water-soluble, emulsifiable, and steam-distillable. A single lipid molecule is small by colloidal standards, but micellization augments the unit particle size to colloidal dimensions. All lipid compound molecules are hydrolyzable in water emulsion with varying degrees of ease. Hydrolysis is accelerated by strong acid and alkali. The enzyme lipase hydrolyzes fatty acid esters to glycerol and fatty acids; lipoxydase oxidizes the unsaturated sites to hydroperoxides. Some fatty acids are hydroxylated, and across the hydrophobic spectrum, lipid polarizability ranges from hydroxylated fatty acids and polyunsaturated monoesters to fully saturated triglycerides.

2. Conjugates Lipids complex amylose to form low-melting, reversible conjugates that prevent starch hydration, inhibit granule swelling, minimize leaching, delay or eliminate retrogradation, deprive it of its ability to gel, reduce digestibility, and make the molecule resistant to (~-amylase (Howling, 1980; Galloway et al., 1989; Biliaderis, 1992). They have a profound effect on wheat-starch gel strength, syneresis, pasting peak, and consistency (Takahashi and Seib, 1988). Starch-lipid complexes are believed to influence the baking characteristics of cereals by elevating Tgel (Belitz and Grosch, 1987). Carrageenan and phospholipids stabilize milk fat by a sulfate-amine reaction (Yalpani, 1988). Allegedly, the "rich" sensation of a 3.5% butterfat milk can be achieved in 1% low-fat milk by adding ~- and K-carrageenan at 0.02-0.04% concentration. The texture and appearance of fluid skim milk ( < 0.5% butterfat) can be improved similarly (Moirano, 1977). Glycerol stabilized a dispersed protein, while fatty acids accelerated its denaturation (Buttkus, 1970). Emulsan, a naturally occurring bacterial lipo-

II. Interactions

107

aminopolysaccharide containing approximately 15% lipid and a protein fraction, is strongly surfactant.

C. Polysaccharide- Metal Many naturally occurring ionic polysaccharides are mixed salts of alkali, alkali-earth, and transition metals with different insolubilities. Salts of alkali metals are invariably soluble. Sodium, the most ubiquitous alkali, possesses a single valence electron, large atomic and ionic radii, and very low ionization potential. Na + hydrates in aqueous solution and retains its coordination water in the solid state. Prior to use, native polysaccharide salts are usually converted to the sodium form whence they acquire functionality. Alkali-earth metals (calcium, barium, and magnesium) complex with polysaccharides extensively (Reisenhofer et al., 1984). Calcium has a smaller atomic and ionic radius than does sodium and, because it has two valence electrons, it is endowed with greater polarizing and bonding ability than Na+. Ca and Ca 2+ easily form insoluble complexes with oxygenated compounds. Polysaccharide salts of alkali-earth metals are generally insoluble. Aluminum is an amphoteric element that acts as a nonmetal in alkali and develops a hydrated gelatinous aluminate of a species [Ai(OH)4] n . As a result of this reaction, certain suspended matter including polysaccharide polyanions coprecipitates by entrainment. This element, applied in atomic or ionic form, is a c o m m o n technique for commercial isolation of pectin. In acid, Al3+ supposedly neutralizes polyanions to yield the aluminum salt. After precipitation of the p e c t i n - a l u m i n u m complex, the metal ligand is removed by acidification and washing. Boron is devoid of metallic character: in water, it generates weakly acidic boric acid [B(OH)3]. This hydroxide bonds covalently with vicinal (neighboring) hydroxyl groups to form negatively charged, acidic complexes. Copper is an element of the transition series, much less reactive than the alkali and alkali-earth metals. It possesses one principal valence electron, but is capable of existing in more than one valence state ( + 1, + 2, + 3) as a result of electron transfer from inner orbitals. The multiplicity of valence states permits Cu to enter into chemical complexation reactions with inorganic and organic compounds alike. Many Cu solutions are colored blue. Fehling's reagent, used to test for reducing sugars, is a water solution of CuSO 4, sodium hydroxide, and a sequestrant (potassium tartrate). The reagent loses color in a positive reaction, e.g., in a test for aldehyde and ketone sugars in which Cu 2+ is reduced to Cu+, as the sugar is oxidized. Iron is a nonamphoteric, transition element with the ability to exist in two oxidation states--Fe 2+ (ferrous) and Fe 3+ (ferric). A positive reaction to alkaline ferric chloride is an indication of the presence of hydroxyl groups with which Fe z+ forms colored complexes. Stable copper and iron chelates

108

5. Additivity, Complementarity, and Synergism

are common in plants. In vitro, Cu and Fe are components of solvent systems for cellulose (Jayme and Lang, 1963).

D. Cyclodextrin and Amylose Clathrates Hydrogen atoms are located on the inside of the cyclodextrin ring, creating hollow hydrophobic interiors capable of electrostatically binding linear, similarly hydrophobic compounds. The interaction is referred to as molecular encapsulation: the cyclodextrin is the "host" and the complexed molecule is the "guest" compound. This host-guest reaction can advance by polymerization to supramolecular structures (Harada et al., 1993). The complexes decompose by heating to 240-265~ Cyclodextrin-lipid complexes are surfactants (Shimada et al., 1992). The amylose helix also bears the pyranose hydrogen atoms on the inside and the hydroxyl groups on the outside of a spiral primary chain, which makes the interior surface strongly hydrophobic and the outside strongly hydrophilic (Freudenberg et al., 1939; Kerr, 1950). As a result, the hollow interior of the amylose helix is conducive to fatty acid complexation. The net hydrophobicity of most of these complexes confers a degree of insolubility and crystallinity on them.

E. Polysaccharide- Protein Like polysaccharides, proteins impart texture and structure to foods. Protein-polysaccharide blends may assume physical, ionic, or covalent character, may be soluble or insoluble, and may sometimes exhibit synergism. The characteristics of the blend are as much a response to the reaction environment as to the properties of each ligand. 1. The Nature and Properties of Proteins

Protein molecules fall within the limits of colloidal dimensions and are therefore subject to the same interfacial forces as polysaccharides. They too are amphilphilic--existing as zwitterions (internal salts) in water at neutral pH and in the solid state, and reacting chemically and physically with ionic and some nonionic molecules, often specifically (Hart et al., 1992). Their complexes have different bond strengths along with possibly different conformations (Dickinson and Euston, 1991 a, b; Mackie et al., 1991). The amino acids comprising proteins are held together by the peptide bond ( - O C N H - ) in open (linear), cyclic (spherical), or branched configuration. Proteins and polypeptides, are more compact and dimensionally stable than polysaccharides, because of the inherent preponderance of ionic and

II. Interactions

109

disulfide bridges. The peptide bond is more resistant to chemical hydrolysis than the glycoside bond, but is nevertheless vulnerable to prolonged heating in acidic and basic media and to enzymes. Advanced hydrolysis yields a mixture of short-chain peptides with various DP. In all its reactions with polysaccharides, the protein may be negatively or positively charged or may have a net charge of zero (at the isoelectric point; pI). Many more neutral and charged polysaccharides do not engage in any visible reaction with proteins (Dickinson and Euston, 1991a). Heating denatures proteins and, like polysaccharides, they decompose before reaching T m . Proteins are protective colloids and emulsifiers more efficacious than polysaccharides. The sulfated polysaccharides are the most chemically reactive with proteins.

2. Polysaccharide-Protein Blends

According to Stainsby (1980), there are three kinds of interactions between a polysaccharide and a protein, viz., electrostatic, specific ion, and covalent. The first mechanism is characterized by pH and electrolyte sensitivities; the second mechanism is also pH- and electrolyte-sensitive and additionally is compositional; the third mechanism is heat-irreversible. The order of acidification can determine whether or not the resulting complex remains dispersed or precipitates. Proteins equilibrate as polycations in acidic media (below pI) where they can be stablized by polysaccharide polyanions (Ganz, 1974). Mixing at an initially high pH fixes the protein with a net negative charge, leading to mutual repulsion and deposition when a polyanion is added at that pH. Too low an initial pH depresses polyionization and flocculates casein; consequently, there may be no reaction. Sulfated polysaccharides form soluble complexes with proteins above and below the pI. Pectate, alginate, and CMC have held proteins dispersed under conditions that might otherwise have caused precipitation (Imeson et al., 1977). Polysaccharide stabilizers, in the order of decreasing thermodynamic compatibility with proteins, are pectin > CMC > alginate > gum arabic > dextran (Tolstoguzov, 1986). The emulsion stability of gum arabic (at constant nitrogen content) was found to increase with the polysaccharide molecular weight (Dickinson et al., 1991a, b). The emulsifying power is attributed to preferential adsorption of the protein moiety that bonds hydrophobically with oil, leaving the hydrophilic polysaccharide moiety free to protrude from the droplet surface into the surrounding aqueous medium (Randall et al., 1988). Through Ca 2+ mediation, polysaccharide-protein reactions can occur above the pI where the two ligands are polyanions (Guiseley et al., 1980; Dalgleish and Morris, 1988; Hart et al., 1992). These anionic complexes are differentiated from complexes of paired opposite charges by their indifference to pH.

I 10

5. Additivity, Complementarity, and Synergism

The covalent bonding of polysaccharide and protein may be induced by heating a mixture under conditions of low a w , below the denaturation temperature (Dickinson, 1993); these are the most heat and freeze-thaw stable protein-polysaccharide complexes (Stainsby, 1980). Although critical concentrations of a polysaccharide may enhance the emulsifying and stabilizing ability of a protein, small additions may have the opposite effect (Cao et al., 1990). The discovery of destabilization by 0.05% xanthan on caseinate led Dickinson and Euston (1991a) to remark that small changes in polymer structure or solvent conditions can easily tip the balance in favor of depletion or bridging flocculation. Polysaccharide-protein combinations occasionally display synergism, as, for example, that between gelatin and gellan (Shim, 1985); this has enormous implications for fabricated fibrous food texture. Some popular combinations are gelatin-agar, casein-alginate, and casein-pectin. Gelatin-agar dispersions are flocculated at pH 3 for use in sherbets and ices (Meer, 1980a); casein-alginate systems make room-temperature dessert gels (Cottrell and Kovacs, 1980). The most important protein-polysaccharide reaction in industrial food processing is that of milk and K-carrageenan (sodium salt); the latter is added prior to evaporation to obstruct casein aggregation, as calcium (Lin, 1977) and casein concentrations increase with water loss. The ordinarily nongelling )~-carrageenan is indifferent to calcium, but develops a gel with casein in cold milk (Guiseley et al., 1980). Cryostabilization (Levine and Slade, 1988) is conceptually a mechanism of insuring stability of a dispersed protein by inclusion of small molecules that raise Tg above ordinary storage temperatures where the protein normally resides in a denaturing environment. A starch hydrolysate offered such protection to frozen, comminuted protein dispersions through this mechanism (Buttkus, 1970). The old thinking on cryoprotection was that water flowed osmotically into the protein micelles, not only creating a microenvironment unfavorable for denaturation, but performing a eutectic role also, with the result that the protein dispersion remained in the sol state, once the storage temperature did not fall below the eutectic temperature.

3. Phase Diagram of a Polysaccharide-Protein Blend

Dynamic systems are characterized by rates of change--the definition of d and 0. dP is positive if a dependent variable increases, whether or not an i n d e p e n d e n t variable increases, and negative if the dependent variable decreases, whether or not an independent variable decreases. At any maximum or minimum, d is zero and dP is constant. Information about the velocity of the change (the rate of change of d) is extracted from the second derivative of the equation that governs the relationship between the independent and dependent variables: this differentiation of d (yielding the second derivative denoted d 2) provides information about maxima and

II.

I I I

Interactions

minima. Phase diagrams are used to portray differential changes in ternary dispersions of water, polysaccharide, and protein. If T of an aqueous polysaccharide (1)-protein (2) dispersion is slowly raised, an upper T c is eventually reached, where the condition is imposed t h a t AG1, 2 < ( A G 1 -Jr- AG 2) and the cosolutes are miscible in all proportions. Below T c, the blend is either in separate phases or in stable, metastable, or unstable equilibrium of partially miscible phases, depending on the concentration range. If there is phase separation, one phase contains a major concentration of the polysaccharide and a minor a m o u n t of protein, and the other phase vice versa. Two such phases in equilibrium are called conjugate solutions (Glasstone and Lewis, 1960). In the mixed dispersion, +1 + +2 = 1, and a graph of AGmix VS +i at each T below T c (Fig. 2) shows two stable concentrations at +'1 and +"a, where AGmix is minimum, and one unstable c o n c e n t r a t i o n a t +1, where AGmix is maximum. Metastability prevails in the

Tc

AG,,,b,.

I

I,

I

I

I

I

I

I

r

r

r

r

r

r

I

1

0

Figure 2 Typical phase diagram of an aqueous polysaccharide (1)-protein (2) dispersion showing the Gibbs free energy as a function of the volume fraction (+) of each, at different temperatures from T 1 , where the dispersion is metastable, to the critical solution temperature (To), where the two components are miscible in all proportions. ABC is the spinodal curve: DBE (not connected) is the binodal curve.

5. Additivity, Complementarity, and Synergism

112

intervening range on either side of +1 between +'1 and +"1. The asymmetry of the lines reflects the size and shape disparities between components 1 and 2. At the A and C minima and B maximum, d(AGmix)/d+a = 0; from +1 = 0 to A on the + axis, d(AGmix)/d+l < 0 (negative), until A; thereafter, d(AGmix)/d+a > 0 (positive) to the m a x i m u m at D. As a result of the negative-to-positive change from 0 to D, d2(AGmix)/dd? 2 > 0 (positive). From D to B, d(AGmix)/d+a remains positive, d declines to 0 at B, and declines further to E (negative slope); from D to E, dZ(AGmix)/dd?~ < 0. From E to C, d(AGmix)/d+a remains negative, but the rate change is positive to +1 = 1, resulting in d 2 AGmix/d+ 2 > 0 from C to +i = 1. D and E are inflection points, where dZ(AGmix)/dd~ 2 = 0, inasmuch as the rate change on one side of each is opposite in sign to the rate change on the other side. For +2 in the opposite direction, the signs are reversed. The conjugate solutions are completely miscible at any concentration at and above Tc . The events typifying Fig. 2 may be stated nonmathematically as follows: an aqueous dispersion of a polysaccharide and a protein not exceeding a total concentration of 4% (Tolstoguzov, 1986) is relatively most stable at the c o m p o s i t i o n s (~'1 and +"1 and least stable at +1 ; metastability can be expected in the vicinity of the concentrations at D and E. The locus connecting the minima ABC in Fig. 2 is the cloud point or binodal curve and DBE (not connected) is the spinodal curve (Cowie, 1991). The events outlined in Fig. 2 are expressed in Eq. (5.3), derived from a series of substitutions and differentiations of AGmix with respect to the solvent's partial molal content (Cowie, 1973, 1991):

d(AGmix)/d(Nno) =RTc[ln(a - + 2 ) + (1 - 1/Xn)+2 + •

(5.3)

X = AHmix/RT~Nno+i"

(5.4)

Nn o is the n u m b e r of solvent molecules, x, is a dimensionless DP function, and X is the Flory-Huggins interaction parameter calculable from osmometry data (Ulrich, 1975): ,n/RTc i = ( 1 / M i ) + [do/(Mod2)](0.5 - •

i+

....

(5.5)

M i and M o are the solute and solvent molecular weights, respectively. The second virial coefficients in Eqs. (4.30) and (5.5) are related via the equation

= [do/(Mod

)] (O5 - •

(5.6)

W h e n there is no interaction, i.e., when [3 = 0, • = 0.5, the m a x i m u m theoretical value. The experience with synthetic polymers is that • approximates 0.5 in poor solvents and is lower in good solvents (Cowie, 1973). Invalid assumptions of the theory spawning Eq. (5.3) are discussed by Cowie (1991).

IV. Summary

I 13

III. Antagonism In antagonism, the combined property imputed to cosolutes in a dispersion is less than the sum of the properties imputed to each solute. If for any reason, e.g., geometry precluding parallelism and identical charge instigating repulsion, the cosolute segments are more cohesively than adhesively attracted, each cosolute may gel independently in a separate saturated phase containing a minor proportion of the other. The two phases may remain a liquid separated by a sharp interface under appropriate conditions of concentration, pH, i, etc. Coacervation is an antagonistic event, according to Dickinson (1993); it differs from thermodynamic instability in that it produces one phase that is almost entirely solvent. Although mutual repulsion can rationally explain antagonism (Robb, 1986), it cannot do so between antagonistic gelatin (positively charged) and amylopectin (zero charge) (Grinberg and Tolstoguzov, 1972), CMC (negatively charged) and nonionic gums (Hercules, Inc., 1980), neutral dextrin and neutral amylose (Kalichevsky et al., 1986), and possibly neutral locust bean gum and low-methoxyl pectin (negatively charged) (Lopes da Silva et al., 1993). Moreover, amylose and amylopectin exist commensally at higher concentrations in starch granules in various ratios, but when isolated, they are mutually incompatible at moderate concentrations of each totaling 2.5 w/w% (Kalichevsky and Ring, 1987) to 4% (Morris, 1990). The amylose:amylopectin range of incompatibility is 15:85-22:78. Below approximately 15% amylose, amylopectin dominates the continuous phase; inversion occurs above approximately 15% (Doublier and Llamas, 1993). The introduction of Na + (> 10 -3 M) or Ca 2+ (5 X 10 -4 M) had an antagonistic effect on the xanthan-guar gum interaction. This antagonism was thought to be due to the cationic screening of the xanthan charge and the resulting conformational change (Clark, 1988).

IV. Summary Polysaccharides interact additively, complementarily, synergistically, and antagonistically with other food molecules. Heated polysaccharide-protein blends tend to be most stable at a low volume fraction of either cosolute; they become increasingly unstable as the concentrations approach equality. The miscibility of phases depends strongly on temperature. Binary and ternary dispersions alike manifest incompatibility by metastability, phase separation, and deposition. Stability can be prolonged by adjusting the volume ratios and storage and processing temperatures within critical limits.

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CHAPTER 6

Thermal Processing I. Introduction The time-temperature integral is the singularly most effective stimulus on the polysaccharide disperse system, from the mildest process that insures safety and elementary dissolution to the severest process that initiates chemical decomposition. On the lower response scale, gelatinization and swelling are primary occurrences; on the upper responsescale, chemical dehydration, pyrolysis, and resynthesis generate higher M species in the involatile phase (Fagerson, 1969) and flavors, aromas (Teranishi et al., 1992), colorants (Vercellotti and Crippen, 1991), and a host of other small organic molecules (Horton, 1965) terminally. Fresh bread, pastries, roasted peanuts, etc., owe their sensory appeal to pyrolysis. The thermally induced changes may not always be beneficial. Polysaccharide decomposition products are generally considered safe for human consumption, but traces of potentially deleterious compounds, e.g., phenol (Byrne et al., 1966) and acrolein (Walter and Fagerson, 1968), may appear. The ~/-lactone of 4-hydroxy-2-pentenoic a c i d - - a vapor-phase constituent of the pyrolysate--has been implicated in antibiosis (Oxford, 1945).

II. Atmospheric and Retort Processing With few exceptions, polysaccharides are processed for food in a moderately high a w environment from 100~ and 1 atm to 121.1~ and > 1 atm (retorting, ultrahigh temperature pasteurization, and extrusion). Mindful of the combined gas laws (ITV/ = n i R T ) , the polysaccharide (n i) undergoing thermal processing in enclosed space (V/) must be subjected to high stress (~r). Heating to high temperatures and pressures in a closed, evacuated container causes desolvation and demixing of the compacted macromolecules, as water volatilizes into the headspace. Desolvation and demixing

115

I 16

6. Thermal Processing

of the food substrate are a condition of --ASmi x (Elias, 1979). Cooling reverses the process. The thermophysical events in a can in the retort (dissolution, hydration, dehydration, gelatinization decrystallization, defibrillation, curling, uncurling, etc.) obviously must be complex. Charge superimposes electrostatic and electrokinetic reactions on the thermophysical processes. Broken-curve profiles for some polysaccharide foodstuffs manifest a transition from conduction to convection heating, as a tenuous, reversible suprastructure reverts to a liquefied mass u n d e r the influence of + AHmi x . The effect of heat on the polysaccharide-water interaction in several dispersions and suspensions was studied by comparative viscometry and rheometry (Tables I-IV). The polysaccharides were the purest manufacturers' grade laboratory washed and dried before dispersion. The dispersion concentrations were below c* to accommodate capillary viscometry, and the suspension concentrations were above c* to accommodate rheometry. It is seen in Tables I and II that the cellulose derivatives made the most stable dispersions and the propylene glycol alginate made the least. Dispersions of the neutral polysaccharides were more stable than those of the ionic polysaccharides. From Tables III and IV, it can be argued that suspensions benefit

TABLE I Ratio of Intrinsic Viscosity of Polysaccharide Dispersions Heated ([TI]E) for 15 min at 12 I~ and the Corresponding Unheated ([~1],) Controls~

Polysaccharide Methylcellulose Na-CMC Hydroxypropylmethylcellulose Hydroxyethylcellulose Konjac gum Locust bean gum Guar gum Xanthan gum Gellan gum Carrageenan Baking pectin Algin Low-methoxylpectin Propyleneglycolalginate

['q]h/[~q]c 1.0 1.0 0.9 0.9 0.9 0.8 0.8 0.7 0.7 0.4 0.3 0.3 0.2 0.0

aThe highest dispersion stability is indicated by unity.

II. Atmospheric and Retort Processing

I 17

T A B L E II Ratio of the Slope of ~i vs c i for Polysaccharide Dispersions Heated ([3 h) for 15 min at 121~ and the Corresponding Unheated ([3c) Controls

Polysaccharide Methylcellulose Na-CMC Hydroxypropylmethylcellulose Hydroxyethylcellulose Konjac gum Locust bean gum Guar gum Xanthan gum Gellan gum Carrageenan Baking pectin Algin Low-methoxyl pectin Propyleneglycolalginate

Bh/ ~ ~

0.8 1.0 1.1 0.5 0.6 0.6 0.5 0.6 0.7 0.2 0.1 0.3 0.0 0.1

f r o m h e t e r o g e n o u s solid-phase stabilization. T h e a p p r o x i m a t e l y 11% CMC coat on Avicel u n d o u b t e d l y a d d e d t h i c k e n i n g p o w e r as m u c h as did hydration of the h e a t - d i s o r d e r e d microfibrils. Rao et al. (1981) r e p o r t e d the primary structure of g u a r g u m a n d CMC to be quite stable after h e a t i n g at 210-260~ for 5 - 2 0 m i n , b u t t h e r e was a loss of nqa. T h e results of this h e a t i n g study suggested possible c o i l - s t r e t c h d e f o r m a t i o n of an e q u i l i b r i u m structure in the gums b e y o n d their elastic limit a n d an infinitely long t I after cooling. C o i l - s t r e t c h d e f o r m a t i o n s can

T A B L E III Ratio of Apparent Intrinsic Viscosity of Heated ([~1 a]h) to Control ([~1 o]c) for Some Polysaccharide Suspensions

Polysaccharide

['qa]h/['qa]c

Avicel FD-100 Avicel RC-591 Amylopectin Corn starch Amylose

2.5 1.5 1.7 3.0 27.4

I 18

6. Thermal Processing

TABLE IV Ratio of Slope (13) of Heated (h) to Control (c) for Some Polysaccharide Suspensions Polysaccharide

[3h/[3c

Avicel FD-100

0.9

Avicel RC-591

1.1

Amylopectin

0.9

Corn starch

7.7

Amylose

1.3

also explain viscosity loss in flow-through (tubular and plate) pasteurization and birefringence. A distinction is made between dispersion stability and chemical bond stability: the former refers to the tenacity of a reversible tertiary structure in its dispersion medium (solid, liquid, or gas) wherein any conformational shift is theoretically transient and finite, albeit in a possibly long interval. Chemical instability involves decomposition of the primary structure of covalently linked glycosides. Chemical bond rupture is irreversible and the decomposition E a is much higher than the Ea of conformational distortions and viscous flow. Raemy and Schweizer (1983) assert the following thermochemical stabilities: (i) 1,4-[3-glycans are more stable than 1,4-et- and 1,6-0t-glycans (ii) glucopyranoses are more stable than fructofuranoses (iii) uronic acids are among the least stable polysaccharides Specifically, D-glucose < maltose < maltotriose < amylose < starch < amylopectin < cellulose (Greenwood, 1967). Trends indicated are that thermochemical stability increases with the DP, branching, and 1,4-[3 bonding. Chemical bonds other than 1,4-et and 1,4-[3 introduce heat and acid instability. Either of these two bonds is less easily depolymerized when the sixth pyranose carbon is oxidized to the carboxyl group rather than esterified; for this reason, low-methoxyl pectin is more stable than high-methoxyl pectin.

III. Low-Temperature Pyrolysis Pyrolysis (Irwin, 1979, 1982; Tomasik et al., 1989b) is the decomposition of a substance at elevated temperatures, principally by dry heat. Low-temperature pyrolysis arbitrarily refers to thermochemical decomposition in the 121.1-

IV. High-Temperature Pyrolysis

I 19

300~ range, where polysaccharides are moderately stable for short time intervals, but, after prolonged heating, there is an initial chemical dehydration, followed by chemical bond rupture that produces lower DP fractions, acids, maltose, isomaltose, glucose, and an increasingly complex vapor phase. Acidity in the involatile phase is an exponential function of time at a constant temperature (Walter and Fagerson, 1970); simultaneously, there is an accumulation of levoglucosan (1,6-anhydro-[3-D-glucose). The decomposition is initially autocatalytic, with a rate increasing from lower to higher temperatures. Metal ions, H 3 0 +, and O H - catalyze the decomposition. Spectrally, the pyrolysate shifts from an ultraviolet-nonabsorbing species to an ultraviolet-absorbing chromophore. The pyrolytic pathways and pyrolysate properties, to varying degrees, are dictated by the substrate, its purity, moisture content, the heating medium, and the time-temperature integral. The most striking chemistry of low-temperature pyrolysis involves condensation, addition, transglycosylation, and the construction of branched polymers in the involatile phase; the reactions begin in the amorphous regions (Major, 1958). High time-temperature integrals generate furans, but in the presence of proteins the vapor-phase composition shifts to pyrazines. The low-temperature pyrolysis of acidified starch (Horton, 1965; Greenwood, 1967) for a relatively short interval (4-8 h) produces "white dextrins" that are quite similar in appearance and function to unheated starch except that their reducing power and water dispersibility are enhanced, and viscosity and the retrogradation tendency decrease. "Yellow dextrins," having even lower viscosity, are produced over a larger time-temperature integral. "British gums" are the product of heating in alkali at 130-220~ for 10-20 h, coinciding with a loss of birefringence and crystallinity beginning at approximately 180~ British gums have greater sol stability and intrinsic viscosity than white and yellow dextrins. Up to 300~ air oxidation does not appear to be significant (Tomasik et al., 1989b). Cellulose reacts similarly to starch at a faster rate in air than in nitrogen (Shafizadeh, 1968).

IV. H i g h - T e m p e r a t u r e

Pyrolysis

Polysaccharide pyrolysis at 375-520~ is accompanied by a higher rate of weight loss and evolution of a complex mixture of vapor-phase compounds preponderantly of H20, CO, CO2, levoglucosan, furans, lactones, and phenols (Shafizadeh, 1968). The volatile and involatile phase compositions are conditional on the rate of removal of the vapor phase from the heated chamber (Irwin, 1979), inasmuch as the primary decomposition products are themselves secondary reactants. The reaction kinetics is described as pseudo zero order (Tang and Neill, 1964) and zero order initially, followed by pseudo first order and first order (Lipska and Parker, 1966), suggesting an

120

6. Thermal Processing

early independence of concentration, but depending terminally on one or more decomposition products of the primary reaction. The flammability of the vapor phase of cellulose apparels gave special urgency to a search for ways to lower the combustion temperatures (lower E a) and increase the rate of weight loss by using flame retardants, thereby rapidly augmenting the rapid accumulation of nonflammable gases at the expense of combustible distillate (Shafizadeh, 1968).

V. Maillard, Amadori, and Strecker Degradations The chemical reactions in a solution or dispersion of a reducing sugar and an amino or imino compound (ammonia, amino acid, polypeptide, protein), aided by heat (known trivially as the Maillard reaction and scientifically as glycosylation), is one form of nonenzymatic browning of carbohydrates. Reducing carbohydrates easily oxidize to carbonyl compounds, and carbonyl compounds easily develop complexes with nitrogen compounds (Hodge, 1953). The Amadori rearrangement and dehydration multiply unsaturation in the glycosamines into brown pigments called melanoidins; these conjugates are antinutritional, because they make essential amino acids metabolically unavailable. The conjugates proceed to decomposition (Strecker degradation) and subsequently to evolution of aroma and flavor compounds, notably the pyrroles and pyrazines, from roasted peanuts, popcorn, chocolate, cocoa, cooked potatoes, beer, and bread (Belitz and Grosch, 1987), for example. Some pyrroles are constituents of coal tarma very carcinogenic distillate of bituminous coal. The chemical composition of the pyrolysate varies with nitrogen source; each Maillard system produces a different composition in the involatile and volatile phases. A starch-glycine mixture, heated at 290~ was significantly different from the starch control in its composition of alkoxyphenols and imidazoles in the involatile phase, and pyrazine, pyridine, methylpyridine, and dimethylpyrroles in the volatile phase (Umano and Shibamoto, 1984). The neurotoxin 4(5)-methylimidazole appeared in the vapor phase when ammonia, but not amino acids, was the nitrogen source (Tomasik et al., 1989b). A corn starch-sucrose combination inhibited the Maillard reaction (Lee and Woo, 1988).

VI. Caramels Industrial caramel, known from the beginning to be essentially a mixture of burnt sugar polymers (Salamon, 1900), arises from the controlled action of heat on dry sugar or concentrated sugar solutions with or without acid,

VII. Summary

121

alkali, ammonia, or an amino compound (Tomasik et al., 1989a). Polysaccharides undergo identical reactions after the initial pyrolytic decomposition. This nonspecific high DP, bitter-tasting, yellow-to-brown class of decomposition compounds is manufactured for artificially coloring food and beverages (distilled liquors, beer, carbonated beverages, soups, and candies, etc.). The contemporaneous chemistry of caramel, not yet completely elucidated, may be summarized as follows: its variable composition is a function of time and temperature; browning first appears at approximately 160~ whereupon the color becomes progressively darker simultaneously with frothing and evolution of a large quantity of water, traces of acetic acid, and a distillate containing furfural (2-furaldehyde); frothing ceases at about 250~ when the pyranose ring structure is completely chemically dehydrated; the residue (caramel) is soluble in water during the initial stages of decomposition, but is increasingly insoluble in direct proportion to the disappearance of the sugar; hygroscopicity and ethanol solubility increase with residual glucose content; when carbonate or ammonia is added prior to heating, the substrate develops the brown color immediately; many consecutive and competitive reactions (Houminer, 1973) yield a vapor phase of diverse compounds (Bryce and Greenwood, 1966; Walter and Fagerson, 1968). At 300~ in a nitrogen atmosphere, the dominant intermediate product was found to be dianhydroglucopyranose (Heyns et al., 1966). One caramel compound was reported to approximate the formula C 2 4 H 2 6 0 1 3 , reaching C125H188080 and higher (Tomasik et al., 1989a). Caramel is unintentionally generated in burnt carbohydrate foods (rice, oatmeal, cornmeal, etc.) and molasses (Kowkabany et al., 1953); it is the source of maple flavor and color in the concentration of maple sap to maple syrup (Stinson and Willits, 1965). In industrial manufacturing, the intended application is taken into account, because reaction conditions help determine the properties of the pyrolysate, e.g., its tinctorial value, water solubility, and alcohol stability. Tinctorial value refers to the absorbance at 560 nm of a 0.1-wt/vol% solution in a 1-cm cell. Tinctorial strength increases with acidity, temperature, and duration of heating. Caramel manufactured above pH 6.3 is biologically unstable and much below pH 3'1, it is a resin.

VII. S u m m a r y Throughout the range of heat processing temperatures, polysaccharides transmute and partially decompose to palatable food ingredients, appealing flavor, aroma, and color additives, leaving resins as an involatile residue. The transmuted matrix is generally recognized as safe for human consumption, although traces of antinutritional and toxic compounds may be generated.

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CHAPTER 7

Isolation, Purification, and Characterization I. Introduction Polysaccharides are physically and chemically characterized in attempts to correlate their structure, properties, and function. Inasmuch as they cohabit space in vivo alongside and interspersed with numerous biochemicals that can be expected to be extracted with them, they must be purified prior to characterization.

II. Extraction and Purification As objects of study, polysaccharides must first be isolated from the vegetative milieu and concentrated in as high a concentration as possible, without serious structural modifications. Protein, lignin, lipid, ash, and shorter-chain congeners, for example, are invariably present in substantial or trace quantities even after rigorous purification. Amino acids and protein are particularly noticeable in commercial samples of a number of gums (Anderson et al., 1986). Absolute purity is seldom achieved; relative purity is enhanced naturally by large granules (in starch) and concentrated deposits (in microbial excretions). With exceptions, polysaccharides are customarily separated from vegetable matter with the use of aqueous reactive solvents (hot dilute acids, alkalis, oxidants, etc.). In the absence of careful control, modifications of the native structure usually attend isolation and purification with these reagents; the extent of modification is proportional to the severity and duration of the exposure. Mineral acids hydrolyze all polysaccharides, given enough strength and an adequate time-temperature integral. Dilute alkali may simply swell polysaccharide fibrils with only minor molecular changes (Jayme and Lang,

123

124

7. Isolation, Purification, and Characterization

1963); strong alkali isomerizes, enolizes, and hydrolyzes glycopyranoses. Starch, for example, is decomposed after enediol isomerization antecedent to excision of simple organic acids (Schoch, 1964). High temperatures expectedly accelerate the processes. Starch may be extracted simply with hot water, but the mildness of this exercise requires separate inactivation of depolymerases (by heat or ethanol) if this biopolymer is to be used for food, or mercuric chloride (0.01 M) if it is to be used other than for food. The crude water extracts are centrifuged, filtered, and spray- or freeze-dried in preparation for storage. Where there is protein, in small samples, most of it can be removed by shaking the crude extract with one-tenth its volume of toluene and discarding the toluene layer. Lipid matter is dissolved by refluxing with aqueous 80% methanol. Formal procedures for starch have been outlined (Badenhuizen, 1964; Watson, 1964; Wolf, 1964; de Willigen, 1964). In one micromethod (Pucher et al., 1948), sugars were dissolved in 80% ethanol and the sugar-free tissue (50-250 mg) was heated in water to gelatinize the starch; the mixture was cooled before adding 52% perchloric acid with stirring; then the mixture was centrifuged. The steps were repeated and the combined centrifugate was decanted. To 10-mL decantate was added 5-mL 20% sodium chloride and 2-mL iodine-potassium iodide reagent (75.5-g 12 and 7.5-g KI in 250-mL water). After mixing and standing for 20 min, the supernatant liquid was decanted. The starch-iodine precipitate was repeatedly suspended in 5-mL ethanolic sodium chloride (350-mL ethanol, 80-mL water, and 50-mL 20% aqueous sodium chloride) and diluted with water. Subsequent to final centrifugation, the washed precipitate was dispersed in 2-mL 0.25-N ethanolic sodium chloride and the mixture was shaken gently until the blue color disappeared. The liberated starch was washed, centrifuged, and redispersed in 5 mL of hot water. Amylose can be further purified by cellulose adsorption chromatography, because there is a strong affinity of cellulose adsorbents for it. The adsorbed amylose is desorbed with hot water. In yet another extraction, highly purified starch was isolated from apples with 1% cold ammonium oxalate, without disintegration of the granules (Johnston, 1956). Methods have been outlined for cotton (Corbett, 1963), bacterial (Hestrin, 1963), and wood cellulose (Green, 1963). For the highest purification, cotton may simply be washed with hot water, dilute acid, or dilute base. Relatively pure cellulose from Acetobacter xylinum was harvested by filtration alone (Ring, 1982). Complex cellulose forms require rigorous procedures frequently involving delignification and bleaching (with chlorite, sulfite, and hydrogen peroxide). The solid remainder after cellulose delignification is holocellulose, from which hemicellulose may be extracted with cold alkali, leaving an insoluble fraction called oL-cellulose (Ikan, 1991). In one outline, cellulose from Valonia ventricosa, an alga, was boiled in excess 1% aqueous NaOH for 6 h, with a change of alkali solution after 3 h; the alkali-treated cellulose was washed with distilled water (Blackwell, et al., 1977), then immersed overnight in 0.05-N HC1 at room temperature. The

III. Analysis

125

primary structure of simple cellulose, emerging swollen but mostly intact from (cold) alkali treatment, is known as alkali cellulose (Green, 1963). In another outline, cellulose was complexed with cuprammonium ions (Nicoll and Conaway, 1943). Lately, laboratory-scale isolation has relied on polar aprotic solvents and solvent systems, e.g., dimethylsulfoxide, pyridine, N,N-dimethylacetamide-lithium chloride, and 1-methyl-2-pyrrolidinonelithium chloride (Baker et al., 1978; McCormick and Shen, 1982; Seymour et al., 1982; Arnold et al., 1994). These solvents have enabled such homogeneous 17 reactions as O- and N-derivatization of cellulose and chitin (Williamson and McCormick, 1994) and selective site chlorination (Ball et al., 1994). Dimethylsulfoxide was the solvent in a homogeneous reaction of cellulose and paraformaldehyde, prior to isolation of purified cellulose (Johnson et al., 1975). In yet another outline, paraformaldehyde enabled superior quality extracts when the parent tissue s were presoaked in this solution (Fasihuddin et al., 1988). The pectin of commerce is the acid or enzyme hydrolyzate of protopectinmthe insoluble parent polymer residing in the cell walls of higher plants, primarily apples and citrus fruits. The pectin is isolated by entrainment in polymeric aluminum hydroxide produced in situ by neutralization of an acidic aluminum salt in solution. The pectin isolate is freed of AI(III) by washing and dialysis in an acidified ethanol-water solvent. Advanced demethylation of extracted pectin with alkali or enzymes yields a series of low DE pectins. In isolation and purification, although water is of course the main solvent, when alcohol is included, it acts as an antidispersant of the hydrocolloidal solute. Acetone is preferable when treating ethanol-tolerant polysaccharides like cellulose derivatives. The most ethanol-tolerant polysaccharides have been precipitated from water by refrigerating a dispersion of each containing a high volume of acetone. In this laboratory, commercial samples of polysaccharides are routinely dispersed in water, then three times the volume of ethanol is added to precipitate them. Samples are finally rinsed with 95% ethanol. For the ethanol-tolerant polysaccharides, acetone dispersions are refrigerated at 0~ until the solvent and solute phases are visibly separate.

III. Analysis Numerous methods are available for polysaccharide analysis, based either on their chemistry, occasionally involving unique fine structures and subunits, or on their response to ambient stimuli, as chain molecules sensitive to 17. Homogeneityin this sense refers to the uniform solubility(dispersibility) in the same solvent to form a single phase.

126

7. Isolation, Purification, and Characterization

altered environments. Most reactions are understandably nonspecific, given the sameness of physical properties and many structural features. In a number of instances, the polysaccharide quantitative and qualitative methodologies share a common reagent or principle.

A. Detection of Charge and the Zeta Potential Neutral and ionic polysaccharides are distinguished from each other by charge on the latter, which originates from dissociation of acidic groups ( - O S O a H , - C O O H , - O P O 3 H 2 ) , complexation with ionic ligands, or adsorption of ions. The identification of charge is predicated on the polyanion's electrical response in electrophoresis and ion exchange chromatography.

1. Electrophoresis

The migration of ionic compounds in an electrical field is called electrophoresis. Relative to the counterions, polysaccharide polyanions migrate to the positively charged pole (anode), while polycations travel in the opposite direction toward the negatively charged pole (cathode); neutral polysaccharides remain at or near the site of the initial placement. Resolution of a polysaccharide mixture exploits mass-to-charge ratio differences, differential solvent compatibilities, and ~. As~the migrating polyanion encounters viscous resistance from the solvent, an electromotive force is generated (streaming potential). The rate of migration is defined as the electrophoretic mobility f~mthe mobile equivalent of ~ [Eq. (3.7)]. A mixture of different mass-to-charge ratio polyanions that have the same or close 1) in one mobile phase may be resolved further by changing one or more of the solvent variables (solvent system, pH, i, etc.). An accurate 11 rests on an independence from particle geometry, an absence of solvent convection currents, and solute-solute interactions. With foreknowledge of the other variables, Eq. (3.7) may be adapted to measure {. Pechanek et al. (1982) determined ionic polysaccharides by 1) migration through polyacrylamide and agarose gels and on cellulose acetate membranes; the polyanions were detected by staining. At the dimensions found in gel micropores, pairs of surfaces create an adsorption potential ({) 3.5 times that created at the same distance from a single surface (Vold and Vold, 1983). In a method of capillary electrophoresis, 1 pg of dextran was dispersed in alkaline buffer containing fluorescein, and the dextran was detected by negative fluorescence (Richmond and Yeung, 1993).

III. Analysis

[ 27

2. A n i o n Exchange Chromatography

Anion exchangers retain polysaccharide polyanions on a positively charged resin (RNH~) in exchange for O H - or C1- counterions in proportion to their charge density; neutral polysaccharides pass through freely. The most densely charged molecules adsorb closest to the sample placement site. Aldonic, uronic, and ascorbic acids, lactones, and N-acetylated amino sugars were separated on sulfonated polystyrene-divinylbenzene, a strong polyanion exchanger (Wheaton and Bauman, 1953). This method is adaptable to neutral carbohydrates without complexation or adsorption, by immersion in strong alkali to ionize the hydroxyl groups (ion chromatography).

3. Conjugation

Dickmann et al. (1989) developed an anion-specific method of determining food gums based on complexation with horseradish peroxidase and color development with benzidine and a protein ligand. Keijbets (1974) used a modified copper acetate-arsenomolybdate assay to distinguish between hexose and hexuronan end groups. An application to carboxyl polysaccharides of the cation-anion complexation reaction involved poly(hexamethylenebiguanidinium) chloride, without eliciting any reaction from neutral polysaccharides (Kennedy et al., 1992). Using hexadecyltrimethylammonium bromide, Cui et al. (1993) obtained two fractions from a mustard polysacchar i d e m a major insoluble and a minor soluble fraction. Polysaccharide polyanions can be complexed with quaternary ammonium cations (e.g., positively charged cetylpyridium) to form differentially soluble salts (Scott, 1965). el. Electroviscosity

An initial negative slope of 'qi vs c i in a dilute water dispersion (electroviscosity) of a polysaccharide is indicative of polyanionic character. Electroviscosity disappears in excess electrolyte solution and is nonexistent in neutral polymer viscosity profiles.

B. Functional Group Identity Infrared spectroscopy (IR) exploits the absorption of infrared radiation in the 400-4000-cm -1 segment of the radiation spectrum. IR is a generally useful method to help elucidate organic chemical structures (Barker et al., 1956), including the identification of ionizable groups. Thus, IR spectroscopy is an indirect means of detecting charge. Polysaccharides are best examined

128

7. Isolation, Purification, and Characterization

as a thin (1-1.0-nm thick) film, made by spreading a 0.05-0.5% aqueous dispersion on a clean smooth surface (e.g., a watch glass), drying the film in a stream of nitrogen, and prying loose the dried film (with a spatula). Residual moisture is brought to an absolute minimum by storing the dried film over concentrated sulfuric acid (sp gr 1.84; relative humidity less than 3%). The fundamental carbonyl ester band stretches and bends in the 1725-1749-cm- 1 region, with overtones in the vicinity of 3430 cm- 1. O H stretch vibrations are in the 3000-3600-cm -1 range. The acidic carbonyl group absorbs intensely at 1736 cm -a, allowing for changes in dipole moments with different neighboring organic groups. Sulfate absorbs in the vicinity of 1240 cm-1, making it distinguishable from - C O O H . IR bands that are most helpful in detection and quantification of modified celluloses have been listed (Barker, 1963). Ambiguities in interpreting a polysaccharide IR spectrum are obviously possible from overlap of absorbing bands, given the similarity and proximity of many subunits in the macromolecule. The absorption band at 3600-3000 cm-1 (OH stretching) can be eliminated by deuteration, which makes it possible to distinguish between OH groups in the amorphous and crystalline states in cellulose where, in the former, disordered regions facilitate the deuterium-hydrogen exchange; the crystalline regions are refractory to the exchange. The ratio of absorptions before and after deuteration can provide a measure of the degree of crystallinity (Barker et al., 1956).

C. Interaction and Conformation

The 1400-800-cm -1 segment of the IR spectrum is sensitive to polymer conformational changes (Belton et al., 1986). In this segment, Wilson et al. (1988) studied ion-sulfate interaction; they reported an increased ion-sulfate affinity and a higher degree of ordering during gelation of K-carrageenan with potassium. By monitoring the 1046 cm -1 band, they followed the course of crystallization of starch from gelatinization to a 21-day storage condition. Conformational changes are easily followed by optical rotation (Hui and Neukom, 1964). Circular dichroism spectroscopy (CD) of polysaccharides (Morris, 1994) exploits optical anisotropy. In a CD instrumental design, the clockwise and counterclockwise rotation of two polarized beams of equal intensity, traversing a 180 ~ path through a chiroptical medium, display a molar ellipticity maximum and minimum. CD is the differential measurement as a function of h. By CD spectroscopy, mixed interchain association rather than nonspecific incompatibility or exclusion was identified as the molecular basis of alginate-polyguluronate interaction (Thom et al., 1982).

III. Analysis

129

D. Polydispersity Polysaccharide size polydispersity transcends decades of molecular weights (Fig. 3 in Chapter 5). Such polydispersity is evaluated by a variety of methods (Barth and Sun, 1991). 1. Electrophoresis

Given that 1~ is a function of mass-to-charge ratio, the number of spots displayed on an electrophoregram is a semiquantitative indicator of the polymolecularity of the parent polysaccharide (Aspinall and Cottrell, 1970). 2. Chromatography

Partitioning by gel chromatography, customarily performed under relatively low pressures (1 atm; low-pressure liquid chromatography, abbreviated to LPLC), has been adapted to separations at much higher pressures in a method referred to as high-pressure liquid chromatography (HPLC). As in LPLC, this latter mode partitions a population of molecules at the interface between a stationary liquid and a mobile liquid, but in specially designed systems that can withstand the high pressures. HPLC enables higher resolution than LPLC. HPLC has been adapted to industrial polysaccharides (Barth and Regnier, 1981), e.g., guar (Barth and Smith, 1981), starch (Kobayashi et al., 1985), polydextrose (Thomas et al., 1990), pectin (Schols et al., 1989), and other anionic gums (Voragen et al., 1982). Baseline separation is limited to DP = 10 (Chester and Innis, 1986), which is more the size of an oligosaccharide than a polysaccharide. Separations to DP = 30 are possible under special conditions (Praznik et al., 1984). In normal polysaccharide HPLC, the most polar molecules with identical ~r are the last to elute in a polar solvent, because of their greater interaction with the aqueous stationary phase. Reverse-phase HPLC is the technique of substituting the LPLC stationary phase with a nonpolar solvent stationary phase and using a less polar mobile phase, with the result that the most polar homologs elute first. Heyraud and Rinaudo (1991) applied reverse-phase HPLC to the separation of low DP anomeric dextrins and Voragen et al. (1982) applied it to analysis of pectin enzyme digests. Thin-layer chromatography (TLC) is another liquid-liquid partition technique applicable to polysaccharides, but in two dimensions. In TLC, the M cutoff boundaries between separated molecules are sharpened, because diffusion is minimized or eliminated in favor of capillary transport. The sample capacity of a TLC plate is in microliters. Resolution is enhanced further at high solvent pressure (Rombouts and Thibault, 1986).

130

7. Isolation, Purification, and Characterization

Field-flow fractionation is a chromatographic method of separating components in a dispersion or suspension traveling parabolically through a narrow, empty channel in which the carrier stream is subjected to a perpendicular force field (thermal, electrical, centrifugal, etc.). Partitioning is based on molecular size, mass, charge, and density, in response to the force field (Giddings et al., 1980; Barth and Sun, 1991). The residence time in streamline flow is shortest for the smallest molecules and particles; i.e., the smallest molecules and particles are the first to elute. The slow response time of the largest molecules and particles enables them to migrate during flow toward the wall of the tube where the velocity is lowest, which therefore makes them the last to elute. Field-flow fractionation has been touted as a speedy, high resolution technique. By applying a steric subtechnique (Giddings et al., 1980), field-flow fractionation was shown to have the potential to separate stable polysaccharide suprastructures in greater than hydrocolloidal diameters. Moon and Giddings (1993) used the procedure to size starch granules into a bimodal distribution of mass greater and less than 10 ~m.

3. Miscellaneous Methods Broad categories of starch granule sizes are possible by fractionation with butanol (Schoch, 1942). This solvent enters the interior of the amylose helix and forms an insoluble inclusion complex. Photon correlators measure scattered light in a sol, equating this with size, and particle counters measure the conductivity or capacitance of dispersed solute; calibration is necessary. From particle diameters, volumes can be calculated, assuming a spherical geometry. Sizes and distribution are reported as histograms (Fig. 1). By laser diffractometry, Okechukwu and Rao (1996a) found that ungelatinized cowpea starch granules had a unimodal distribution with a mean of 19 ~m. In an unrelated method, Chuma et al. (1982) used photography, a digitizer, and a microcomputer to calculate the size, surface area, and volume of grains and soybeans.

IV.

Molecular

Weights

a n d Sizes m

Molecular weights are determined by end-group analysis (Mn), membrane osmometry (Mn), viscometry (My), size exclusion chromatography (Mw), light scattering photometry, and sedimentation (Mw). Any molar mass computed by these methods must be evaluated critically, in view of a dependence on methodology.

IV. Molecular Weights and Sizes I--IOF~|

13 ]

I=~A L_A--9OO P A R T I C L E SIZE D I S T R I B U T I O N

D i s t r i b u t i o n Graph Sample Name:CORNELL I0..

[cu#2-a ] UNIV., P E C T I N

Oct/21~

ANALYZER

#2

ID#:921020-536

'92

I00

. .

F%

U%

0.05

z

o.1

I0 D i a m e t e r [ #m

C o n d i t i o n s for M e a s u r e m e n t A g i t a t i o n Speed : 3 U - S o n i c W o r k Time: 0min S a m p l i n g Times : I0

% on D i a m e t e r

90.0% 95.0~

Specific S u r f a c e Area = 27.093#m 62.803#m 209.380#m 314.892#m 431.959#m

= =

300.00#m = : :

4 0sec

0.00i

Data: Median Diameter=125.045(#m) D i a m e t e r on % [I] 10.0% = [2] 25.0% = [3] 75.0% =

Transmittance(He-Ne) Transmittance(LAMP)

1000

C i r c u l a t i o n Speed : W a i t i n g T i m e after U-Sonic:

Form of D i s t r i b u t i o n : 1 R . R e f r a c t i v e Index : 1.19 D i s t r i b u t i o n Base : Volume

[4] [5]

I00

l153cm2/cm3

88.8%

79.5% 87.0%

Figure I Size distribution of a sample of pectin (Hercules, Inc., Wilmington, DE). Courtesy of Horiba Instruments Inc., Irvine, CA.

A. ReducingEnd-Group Analysis Reducing end-group analysis (Smith and Montgomery, 1956) is implicitly the most direct way to determine molecular weights of linear polysaccharides, because a chemical reaction establishes one-to-one correspondence between

132

7. Isolation, Purification, and Characterization

each polymer molecule and a reagent molecule, with the result that the measurement is theoretically indifferent to mass and polydispersity, relying solely on a theoretical, exact, but from practical experience, not necessarily stoichiometric quantitation between the two reactants. There is a DP sensitivity limit of approximately 2.5 X 104 Da (Garmon, 1975), suggesting polysaccharide oligomers to be most amenable to this method. Comparison of the results with o t h e r methods is good to poor (Launer and Tomimatsu, 1959). Branched polysaccharides do not present one-to-one correspondence, consequently making reducing end-group analysis unsuitable for amylopectin and glycogen. The molar ratio of polysaccharide reducing end groups to nonreducing end groups can be determined alternatively by methylation (Ingle and Whistler, 1964) and periodate oxidation (Mehltretter, 1964; Shasha and Whistler, 1964; Whelan, 1964). A c o m m o n procedure for starch is oxidation to the dialdehyde, whereby each mole of nonreducing and reducing end group yields a definite n u m b e r of moles of formaldehyde and formic acid. From the data, a ratio of nonterminal to terminal glucose m o n o m e r s can establish the DP, M n , and branching. DP heterogeneity results in wide deviations from the mean. /

B. Viscometry and Rheometry Single-point viscometers allow o n e ~sp/Ci measurement at a time. Dilution viscometers allow for continual dilution in the cell by adding solvent calculated from the equation Vie 1 -- ( g 1 --b 7))c 2 .

(7.1)

V1 is the volume of sample at concentration r and v is the volume of solvent to be added to accomplish the next dilution to c2 . Nine or ten dilutions should be made, but no less than three, so t h a t Tlsp/r i VS r contains a m i n i m u m of four points, excluding c i = 0 where extrapolation yields [~q]. In either single-point or dilution viscometry, the basic "qrel is measured first, precedent to the secondary equations in Table I in Chapter 4. Having plotted and extrapolated ~qsp/Ci vs c i to [xI] and knowing K and v beforehand, substitutions may be made in the M a r k - H o u w i n k equation [Eq. (4.47)]; M v can thus be calculated. By coordinate orientation, small [TI] differences are exaggerated by the exponent in Eq. (4.7), making coordinate orientation suitable for evaluating functionality and making possible a studied selection for a particular use. If, for example, a polysaccharide giving a low viscosity in an ethanol solution is required, C or SS(2) is the pectin of choice (Fig. 2). The response to ethanol is shown to be pectin-specific rather than class-specific. Instead of ethanol concentration, the abscissa may be c i (Walter, 1991).

IV. Molecular W e i g h t s and Sizes

133

28.0 -

26.0 -

24.0 -

22.0 -

20.0

18.0

16.0 Z 14.0

[1] ]2k'

12.0

z&

LM ALM

v~s

=) S S ( 1 ) o o SS(2) uE---I C

10.0

8.0

6.04.0 i ( 2.04

0.0

I

I

I

I

5.0

10.0

15.0

20.0

ETHANOL (v/v%) Figure 2 Viscosity profile of selected samples of pectin in aqueous dispersions, as a function of ethanol content.

[ 34

7. Isolation, Purification, and Characterization

A rheometer measures higher dispersion concentrations than a viscometer and, unlike the latter, can also measure suspensions.

C. Size Exclusion Chromatography The first fraction of solute emerging from a gel column is eluted in v 0 that contains molecules too large to enter the gel micropores, v 0 is the macroscopic pore space in the gel bed, not otherwise participate in the sieving mechanism. For a given column, v 0 is constant. The solute interacting with the liquid stationary phase on the column surface elutes in order of the magnitude of a fraction's partition coefficient (Kp) between the elution volume (Vel) and the volume of stationary solvent in the micropores (vs), fixed at 100 mL, because of difficulty in its measurement (Bio-Rad, 1971). At V~- 100 mL,

Kp=

(Uel- v 0 ) / ( 1 0 0 -

v0)..

(7.2)

It is self-evident from Eq. (7.2) that the smallest sizes possess the largest Kp, because they are the last to emerge from t h e c o l u m n (largest Vel). Kp is relatively large also, if there is bonding between the solid support and the eluting solute, for the obvious reason that bonding extends the retention time and hence l/el. Kp is variable with temperature. The steepness of the slope of an ideal straight line Kp vs log M is an index of the resolution efficiency of a gel bed: the steeper the slope, the higher is the_ baseline resolution. If Kp is plotted ag__ainst the logarithm of standard M of a homologous series, an unknown M should ordinarily be determined from the plot, but for the difficulty in procuring a homologous standard series and the susceptibility of Kp to deviations arising from interaction (Rollings et al., 1983; Anger and Berth, 1985), charge, and polymolecularity. The difficulties presented by log Kp vs log of standard M are obviated by alternatively plotting log[xl]M vs log Vel. For each fraction of a polysaccharide homologous series, log[~l]M vs log Vel is superimposable on a socalled universal calibration curve (Grubisic et al., 1967) from which an unknown M 2 may be estimated, after substitution in the equation

[,q]lM1 "-[T]]2M 2 .

(7.3)

[qq]l and M 1 are values of a standard, e.g., dextran, and ['1]]2 and M 2 are values of the unknown. Berth and Lexow (1991) measured pectin M by

IV. Molecular Weights and Sizes

135

using Eq. (7.3). Barth (1986) derived Eq. (7.4) by combining v and K of the Mark-Houwink equation [Eq. (4.47)] with Eq. (7.3) (Appendix 6): l o g M 2 = [(v I + 1 ) l o g M 1 + log K,

-log

K z ] / [ v 2 + 1].

(7.4)

Size exclusion chromatography did not differentiate lower molecular weight, extended coils from higher molecular weight, compact coils (Berth,

1988).

D. Membrane Osmometry In membrane osmometry, use is made of the fact that the chemical potentials of water (1~0) and of dispersion (tzi), separated by a membrane permeable to water only, are unequal except at equilibrium, and that ~r (a function of AI~) is directly proportional to M , . Accurate measurements are confined to a practical upper limit of 105-106 Da (Garmon, 1975; Dautzenberg et al., 1994). Membrane osmometry is executed in a static or dynamic mode. During static osmometry, preequilibrium aging may decrease the number and increase the average size of polysaccharide particles. Dynamic osmometry rids the measurement of aging as a source of error, but the small volumes of sample used are prone to air saturation that distort the ~v readings. Aggregation lessens the number of colloidal units and consequently lowers M n ; disaggregation has the opposite effect. Semipermeable membranes present problems arising from flexibility of the membrane, from outward migration of molecules in the lower colloidal range, and from circumferential solvent diffusion. The starting electrolyte concentration should be equal on both sides of the membrane to avoid initial pressure surges. The "ballooning" of flexible membranes gives a false reading of ci [i.e., g/V/; Eq. (4.29)]. The outward migration of the smallest particles across the membrane artificially decreases the Mn and narrows the distribution range. Circumferential solvent diffusion results in loss of an increment of -rr in proportion to the weight of escaped water. Donnan distribution, leading to abnormally high ~v, is a most serious source of error for M, of polyanions measured by osmometry. The Donnan effect is allegedly overcome by very dilute concentrations that never exceed 25 g L -1 for the polyanion and i = 0.3 mol L-1 for the solvent (Wagner, 1949). A 25-g L -1 polysaccharide concentration is in the semidilute-toconcentrated domain, outside the theoretical dilution limit of osmometry where a power-law dependence of r i on ci is expected. Equations (7.5)-(7.11) illustrate the effect of Donnan distribution on osmometry. Recalling Eq. (4.29) (ITV/= n i R T ) , assuming an equivalent weight

136

7. Isolation, Purification, and Characterization

of an ionic polymer (i) is in equilibrium in water with three equivalent counterions (1, 2, and 3), and inasmuch as "rr is a colligative property, the number of dispersed particles N n i is m

Nn i = Nci/Mn,

(7.5)

Nn = N ( n i + n 1 + n 2 + n~),

(7.6) (7.7)

= ( N R T / V i) ( c i / M n + C l / M 1 + c 2 / M 2 + c ~ / M ~ ) .

If the counterion is H + originating from dissociation, =

+

Cl/1].

(7.8)

If the counterion is Na+, letting ~ be the equivalent of Na +C1- diffusing to the polyanion ( p 3 - ) from an outer volume at y molar concentration, and r be the equivalent of H + not diffusing outward, the equilibrium condition is p-3 + g N a + + g C I - + r

+~ (y-f~)Na++(y-q~)Cl-+(3-r

+

(7.9)

and for M , , m

m n =ciNRT/('ITVi) + NRT/('rrVi)[(r = ( N R T / ~ r V i ) ( c ~ + r + ~/23 + ~/35). 9

) + (~/23) + (~/35)]

(7.10) (7.11)

m

Relative to c i / M " , r ~/23 and ~/35 are high molar contributions to M , . Notwit__hstanding the problems associated with the Donnan distribution, a pectin M n was obtained from ci = 10 -2 g mL-1 dispersion in 0.05-M sodium chloride and reported to have approximated Mn by reducing end-group analysis (Fishman et al., 1986). An osmometry M , of a standard and an unknown polysaccharide in the same solvent system may be substituted with [-q] in the Mark-Houwink equation, and K and v may be calculated from two simultaneous equations.

E. Light-Scattering Photometry Light-scattering analytical methodology is plagued by the effect of extraneous particles, leading to erroneously large chain dimensions (Jordan and Brant, 1978). In measuring I and I , , the sample must therefore first be freed of extraneous particles (dust, fiber, globules, etc.); this is accomplished by terminal filtration directly into the measuring cell through a 0.45-nm filter (Berth, 1992). Approximately 5% of a pectin solute was discovered in this laboratory to be retained by a 0.45-nm hydrophilic acrodisc (Gelman Sciences, Ann Arbor, MI 48106) and approximately the same quantity to be

V. Colorimetryand Spectrophotometry

137

10st by ultracentrifugation. There is the possibility that spontaneous aggregation in a polysaccharide sol may be the source of the fraction removed. A polysaccharide light-scattering M can differ from a reducing end-group M by more than threefold (Veis and Eggenberger, 1954) and an osmometry M by more than twofold (Walter and Matias, 1991). Dynamic light scattering, coupled with modern computer programs and auxiliary equipment, automatically graphs Zimm plots and computes M w , Rg, ~, f~, and flow rates. New techniques have expanded the method to the semidilute and concentrated regimes (Barth and Sun, 1991).

F. Sedimentation Equilibrium and Sedimentation Velocity In a sedimentation equilibrium experiment, a straight line for Eq. (4.76) is indicative of size homogeneity; curvature is in relative proportion to the degree of molecular weight and size heterogeneity. The experimentation time is considerably shortened by inserting co measured at the meniscus (x 0) and c1 measured at the bottom of the cell (x 1), assuming that the concentration gradient at these two locations is invariant after a short interval (Scholte, 1975). Horton et a l . (1991) measured M w of sodium alginate by sedimentation equilibrium and showed that an accurate determination required inclusion of A s [213 in Eq. (4.8)], even at very low concentrations (2.5 mg rnL-a). In o n e of the very few instances in which sedimentation velocity was applied to polysaccharides, Sharman et a l . (1978) determined M w of several galactomannans in different mannose:galactose ratios and found an independence of the Mark-Houwink equation from this polydispersity. From sedimentation data, in conjunction with data from other methodologies, Harding et a l . (1991b) calculated the mass per unit length of citrus pectin fractions approximating 430 g mo1-1 nm -1, which suggested a rod or wormlike coil with a long persistence length. Wedlock et a l . (1986) found good agreement between a sedimentation and a laser light-scattering M w .

V. C o l o r i m e t r y and S p e c t r o p h o t o m e t r y Unsubstituted polysaccharides do not appreciably absorb ultraviolet and visible radiation, but they can be made to do so intensely by combining them with chromophores and chromogens (e.g., oL-naphthol, dihydroxynaphthalein, anthrone, carbazole, phenol-sulfuric acid, 2-thiobarbituric acid, toluidine blue, diphenylamine, Congo red, aniline blue, and methyl orange), usually in acidic or basic media. Coloration is normally preceded by depoly-

138

7. Isolation, Purification, and Characterization

merization, deesterification, enolization, and dehydration to oligomers, shorter-chain hydrolysates, anhydro rings, and double bonds. There are color reactions for general classes of carbohydrates (Dische, 1962), e.g., those in Table I summarize a few general-purpose tests for routine differential staining of plant-tissue isolates. Chlorozinc-iodine, made by adding a solution of 10 g KI and 0.15 g 12 in 10 mL water to 90-100 mL of a 60% ZnC12 , can identify lignin in a plant extract by its yellow-to-brown color. Cellulose is distinguished from noncellulose matter by its blue-to-violet color (Greenish, 1923). Starch interferes with the reaction, similarly turning blue-to-violet. The color disappears at elevated temperatures and reappears upon cooling. Uranyl acetate is a negative stain for crystalline cellulose; observed under an electron microscope, it shows crystallites inhabiting the translucent areas surrounded by stained amorphous cellulose (Heyn, 1966). Polysaccharides develop color with alkaline hydroxylamine (NHzOH) and Fe 3+. The reagent is made by combining 4 g NHzOH, 14 g NaOH, and 10 g FeC13 in 100 mL 0.1 N HC1 diluted 1:3. The alkali deesterifies the polysaccharide, and the deesterified molecules then develop color with Fe(III) (Doesburg, 1965; McReady and Reeve, 1955; Bean and Bornman, 1973). Starch in helical conformation is indicated by the blue color developed with iodine. In a typical test, a tissue or extract is submerged in a KI3 solution. DP heterogeneity is a factor; the higher the DP, the higher is the 12 absorption and the more intense is the violet-blue color. The starch-iodine reaction is sensitive enough for starch to be a titrimetric indicator of I 2 , mindful of the nonstoichiometry and nonspecificity of the reaction. The I z-starch reaction is the basis of a spectrophotometric assay at 640 nm with amylose standards. A similar Congo red method is less precise than the iodine method, but it has the advantage of insensitivity to the DP, molecular size, and shape. The Congo red assay can therefore be suppleTABLE I Color Reagents and Tests for Some Polysaccharides

Test Chlorozinc-iodine

Cellulose

Lignin

b-r-v

y-b

b

y-b

NH2OH-FeC1 ~ I2 I2-H2SO 4 Carbazole-H 2SO 4 Ruthenium red

g

Phloroglucinol b, blue; r, red; v, violet; y, yellow; p, pink; g, gray.

Starch

Pectin

V. Colorimetry and Spectrophotometry

139

mentary to the iodine assay, to minimize errors caused by heterogeneity (Carroll and Cheung, 1964). The amount of 12 complexing with amylopectin (and glycogen) is much less than with amylose, and the I2-amylopectin complex is red-to-brown (Kerr, 1950). The intensity of the different colors with amylose and amylopectin can be used to differentiate waxy starch from ordinary starch. Of a series of polysaccharides tested, xanthan gum was the only one that reacted positively to toluidine blue and methylene blue (Nakanishi et al., 1974); the latter dye reacts with polysaccharide polyanions including carrageenans, with which it develops a purple color. Quantitative spectrophotometric methods for pectin utilize carbazole (diphenyleneimine; Bitter and Muir, 1962) and m-hydroxydiphenyl (Kintner and Van Buren, 1982). The intense red-to-brown color with carbazole in sulfuric acid, relatively specific for uronans (pectin and alginate), is much less intense with ketohexoses, aldohexoses, and pentoses (Snell and Snell, 1953). The m-hydroxydiphenyl assay is subject to less interference than the carbazole assay. Ruthenium red (ammoniated ruthenium oxychloride) is a strong indicator of the polycarboxylic acid groups in pectin. This reagent is made by adding enough ruthenium red powder to 10% lead acetate to produce a wine-red color. Starch, pectin, cellulose, and cellulose derivatives are assayed with anthrone (Viles and Silverman, 1949; Samsel and Aldrich, 1957), and methylcellulose is assayed with diphenylamine (Kanzaki and Berger, 1959). 2,7Dihydroxynaphthalein (2,7-naphthalenediol) develops a blue-red color with glycolic acid abstracted from the carboxymethyl group of acidified CMC (Graham, 1971; Harris et al., 1995). Allen et al. (1982) developed a specific test for 3,6-anhydrogalactose in K-carrageenan in a mixture of food gums by using 2-thiobarbituric acid. The Maillard reaction with cysteine and methylpentoses (Dische and Shettles, 1948) has been resurrected by Baird and Smith (1989a) and Graham (1990) in a specific assay for gellan in which there is 6-deoxyhexose (rhamnose). Aniline blue, reported to be a specific stain for 1,3-[3-D-glucans, has been adapted to the quantitative analysis of gums containing this structure (Nakanishi et al., 1974). The SO 2- ion stripped from carrageenan was precipitated with Ba 2+ in a gravimetric method of carrageenan analysis (Hansen and Whitney, 1960). Comprehensive methodologies for a large number of other food gums have been outlined (Smith and Montgomery, 1959; Glicksman, 1969; Graham, 1977). Newer assays involve the action of enzymes, e.g., the assay of Baird and Smith (1989b)who treated galactomannans with galactose oxidase to generate H202 that in turn oxidized 0-tolidine in the presence of preoxidase. These authors claimed this double oxidation reaction to be specific for the galactosyl monomer and its derivatives, based on the exclusive oxidation at the C-6 position. 0-Phenylenediamine can be substituted for 0-tolidine; the color is measured at 425 nm.

140

7. Isolation,Purification,and Characterization

In the current AOAC (1990) method of starch analysis, samples are ground and freed of simple sugars with hot aqueous 80% ethanol, extracted with boiling water, and subjected to the action of glucoamylase for conversion to glucose prior to addition of 0-dianisidine; color is measured at 540 nm against glucose standards. Ethanol is a glucoamylase inhibitor, so all traces must be removed. A two-enzyme procedure for the cereal grains, with glucoamylase and glucose oxidase in sequence, has been assigned first-action status. In this case, color is developed with 0-dianisidine; previously, color development was by ferricyanide reduction (AOAC, 1980). Enzymes have been used to differentiate broad categories of polysaccharides. Peroxidase differentiates polyanions from neutral molecules on the principle of complexation of the polyanion with a protein cation and a positive reaction to a protein-specific stain (Dickmann et al., 1989). An immunoassay for the detection of galactomannans (locust bean and guar gums) in the range of 10 ng mL-1 was developed by Patel and Hawes (1988). The gums were captured on an immobilized lectin that specifically bound galactose; a peroxidase oxidized the substrate to a chromogen and measurement was made at 490 nm. Some enzymes identify specific structures, e.g., in the microassay of Ostgaard (1992) for alginate (0.01-1 mg mL -1) in which the molecule was split at the nonreducing end with a lyase and the concomitant unsaturation was measured by ultraviolet absorption at 230 nm.

VI.

CD and NMR

Spectroscopy

The circular dichroism (CD) spectrum of a number of polysaccharides is close to that of the corresponding monosaccharides (Morris, 1994). CD information gleaned from monosaccharides and extrapolated to polysaccharides is valid, insofar as mono- and oligosaccharides are representative of the complete polymer structure. CD applications to polysaccharides have been reviewed (Johnson, 1987). With the use of CD and complementary instrumentation, Morris (1976) studied xanthan insensitivity to salt and temperature, and observed that stability was introduced by a folding back of side chains around the main chain. Dentini et al. (1991) outlined the use of CD to measure the average charge density of pectin chains. Nuclear magnetic resonance (NMR) spectroscopy is routinely applied to small carbohydrate molecules. NMR spectroscopy is based on the principle that radlofrequencles are absorbed by hydrogen and carbon atoms ( H and XSc) spinning in one of two directions (spin quantum number + 1/2) in a magnetic field. In liquids, absorption is recorded as sharp peaks. The frequency displacement (chemical shift) is a function of the 1H and 13C surroundings. + AE is proportional to the number of photons absorbed between these two quantum states, correlating well with anomeric and 9

9

1

VII. Thermal Analysis

141

conformational changes. NMR spectroscopy was primarily a liquid-state method, but newly developed techniques (Segre and Capitani, 1993) have permitted solid-state structures and dynamics to be studied in great detail. Solid-state spectra are sometimes difficult to interpret, because of peak broadening. By NMR spectroscopy, polysaccharide structure (McIntyre and Vogel, 1993) and heterogeneity (Cleemput et al., 1993) have been studied. Kasai and Harada (1979) assigned a helical conformation to heated (60~ curdlan gels with DP > 49; Morgan et al. (1992) studied the relationship between bread staling and starch crystallization; Cooke and Gidley (1992) discovered that the double helix is more prominent in native starch than crystals and that both structures are concurrently disordered during gelatinization; Morrison et al. (1993) showed that lipids are present as inclusion complexes with V-amylose. NMR spectroscopy is quite conducive to the study of water relations. Radosta et al. (1989) discovered that the amount of monomolecular water bound to maltodextrin was independent of state (sol or gel), but was slightly dependent on temperature. NMR spectroscopy has been adapted to the study of relaxation, because the excited atoms return to their respective ground states at definite rates, depending o n t h e nuclear environment (Gidley, 1992). Harris et al. (1995) presented a synopsis of the potential use and limitations of NMR applied to polysaccharides.

VII. Thermal

Analysis

Physical and chemical events are accompanied by ___AH [Eq. (3.12)] in the form of heat loss or gain (+ACp, v) that is measurable by a number of instrumental techniques. Differential thermal analysis (DTA) quantitatively relates ACp v to AT (the difference between the reference and sample temperatures). DTA suffers the handicap of being influenced by the different heat conductivity and bulk density of the sample and reference (Cowie, 1991). In differential scanning calorimetry (DSC), electrical energy from an independent reference and sample heater, necessary to maintain the reference and sample at the same temperature, is measured. Modern instrumentation and computer software make DTA and DSC virtually indistinguishable. In an experimental DTA and DSC design, a reference and a sample are heated and cooled at a constant or variable rate; calibration is necessary. The weight loss during a pyrolytic change may be measured by thermal gravimetry (TG). Thermal transitions and the characterizing intensities are indicated by inflections on a thermogram in a positive or negative departure from the

142

7. Isolation, Purification, and C h a r a c t e r i z a t i o n

baseline of ACp,v vs AT or At, with different magnitudes of maxima or minima whose exact horizontal position depends on such factors as concentration, granule stability (Liu and Lelibvre, 1992), and the method of sample preparation (Morita, 1956). Time is introduced by scanning, and timedependent phenomena, e.g., transport and relaxation, make d(AH) the sum of partial derivatives (Fig. 3; Provder et al., 1983):

d ( A H ) / d t = (O(AH)/Ot)T + ( O ( A H ) / O T ) t dT/dt.

(7.12)

dT/dt is the experimental scanning rate. The maxima [d2(AH)/dt 2= - 0 ] and minima [dZ(AH)/dt 2-- 4-0] are in a positive or negative direction, measured from a common, interpolated, or approximated baseline (Figs. 3 and 4). The area under the curve is proportional to AH. Equation (7.12) is the forerunner of an equation that enables the calculation of E a from dynamic DSC data (Provder et al., 1983). DTA, DSC, and TG have become routine for monitoring polysaccharide solid-state transformations. Examples of important applications are hydra-

a

RESPONSE OF REACTION MIXTURE PLUS PANS

!

i 0

" RESPONSE OF EMPTY PANS

t 5

TIME

t 10

(rnin)

l 15

I 2O

O

t~ O

E

I 100

I, I 200 300 TEMPERATURE

I 400 (~

I

500

Figure 3 Characteristic DSC exotherm (a) and endotherm (b). (From Provder et al., 1983. Reprinted with permission.)

143

VII. Thermal Analysis

0 9 ,1,--(

9 O

I

60

I

70

I

I

I

80

90

100

TEMPERATURE

(~

Figure 4 Differential scanning calorimetry endotherms (DSC model 2910, TA Instruments, New Castle, DE) of cowpea protein-corn starch blends in different ratios (R) at pH 7 and scanning rate 5~ min -1 . (Courtesy of Okechukwu and Rao, 1996b.)

tion-dehydration, solation-gelation (Krag-Anderson and Solderberg, 1992), glass transition, cold crystallization, melting, liquid crystal ordering, dehydration, rearrangement (Morita, 1956, 1957; Hatakeyama et al., 1989), synergism, and antisynergism (Nishinari et al., 1992), usually involving starch systems (Leli~vre, 1992). Morita (1956, 1957) discerned characteristic endotherms in the vicinity of 130 and 255~ for polysaccharides containing predominantly 1,4-~anhydroglucose linkages, and at 340~ for the [3-linkages. Endotherms of oL-anhydropolysaccharides occurred at lower temperatures than did those of [3-anhydropolysaccharides. The major exothermic reaction (decomposition) occurred at 490-510~ A specific thermal curve could not be assigned to the 1,4-(x linear or the 1,6-c~ branched linkages. The thermograms of dried and rehydrated rice starch suggested a dehydration-rehydration cycle initiating modest irreversible conformational shifts. By TG, endothermic changes in starch at 150~ were shown to be due to loss of capillary and absorbed water and to anhydro ring formation at 200-220~ At 270-310~ there was sudden evolution of decomposition gases. Air oxidation seemed not to have been a factor up to 300~ (Tomasik et al., 1989b). Tang and Neil (1964) demonstrated a net endothermic response of cellulose decomposed at 300~ in an inert atmosphere, and a net exothermic response in oxygen. Gekko et al. (1987) used DSC to show a degree of substitution (DS) effect of

144

7. Isolation, Purification, and Characterization

sugars and polyols on the gelation of K-carrageenan. The endotherms in Fig. 4 show a slight upward shift in corn starch Tgz , as a result of protein additions; the protein denaturation AH was much lower than the starch gelatinization AH (Okechukwu and Rao, 1996b).

VIII. Thermodynamic Variables DSC data are recorded on a thermogram delineated by ACp,v vs AT Or At. Inasmuch as it is the energy consumption that is recorded, the thermogram shows + AE proportional to the change in property. DSC is therefore a method of directly quantifying ACp,v, AH, AS [Eq. (3.16)], and ACp,~/g (the specific heat). Exothermic reactions are characterized by positive peaks on the thermogram, and endothermic reactions are characterized by negative peaks.

IX. Structural Elucidation Many of the foregoing analytical methods fit a strategy for elaborating polysaccharide gross structures and extracting information on intra- as well as intermolecular associations. In solid-state structural characterizations, fragments are isolated and correlations are attempted with intact structures (fingerprinting). It is not always possible to assign a particular fragmentation pattern to a specific molecule, but the distribution pattern of the fragments, generated under controlled conditions, can nevertheless provide useful information on p h e n o m e n a like stability (Fang et al., 1981; Liebman and Levy, 1983), complexation, and chain organization (Biliaderis, 1992). Pattern recognition is feasible, insofar as the vapor phase is rapidly flushed from the reaction chamber, so that a pyrogram, as much as possible, is a record exclusively of primary reaction products (Irwin, 1979). Polysaccharides containing the same repeating dimer, e.g., starch and cellulose, are virtually identical (Bryce and Greenwood, 1966; Sjoberg and Pyysalo, 1985) and therefore have virtually identical fragmentation printouts. Gas chromatography (GC), also known as gas liquid chromatography, resolves mixtures of volatile compounds in a high-pressure mode in a heated gas stream, by partitioning solute between a stationary liquid phase adsorbed on a solid support and a mobile gas phase passing over the stationary phase. GC is adaptable to a pyrolysis vapor phase of simple sugars, oligo- and polysaccharides, and uronans that can be made volatile through chemical derivatization (e.g., methanolysis and silylation; Ha and Thomas, 1988).

IX. Structural Elucidation

145

Wheat pentosans were dehydrated to furfural with concentrated HC1, and the furfural was dissolved in dibutyl ether in preparation for GC (Folkes, 1980). This method is claimed to be specific, accurate, and precise for pentoses and pentosans (Folkes, 1980), because 5-hydroxymethylfurfural is not produced, as is the case in hexose decomposition. Previously, furfural was distilled and analyzed colorimetrically. Alkalization increases the yield of 1-, 2-, and 3-carbon, vapor-phase compounds at the expense of anhydro sugars and furans (Ponder and Richards, 1993). Hydrolysis-GC is adaptable to the estimation of DP (Morrison, 1975); applicability to carbohydrates is limited to oligomers with DP = 6 (Traitler et al., 1984). In preparative GC, each separated fragment or derivative passes through a nondestructive detector, or a minor portion of the effluent stream is diverted to the detector and the remainder collected. The analytical advantages of pyrolysis (PY) and GC have been programmed into a sequential method of solid-state characterization of polymers. Morita et al. (1983) decomposed homoglucans anaerobically with heat in alkaline sodium sulfite in the presence of 0-phenylenediamine to form the respective quinoxalines. Previously, it was necessary first to hydrolyze the homoglucans completely and then silylate the monosaccharides. Each class of glucan quinoxaline decomposed to a specific dicarbonyl compound for which the phenylenediamine reaction was specific. Hicks et al. (1985) offered a cation-exchange option to traditional polysaccharide anion-exchange chromatography and GC. Supercritical fluid chromatography (SFC) is a GC method of analysis of compounds in systems where normal GC presents resolution difficulties (Lee and Markides, 1987). A supercritical fluid has properties at a critical temperature intermediate between a liquid and a gas. At and above this critical temperature, a gas cannot be compressed into a liquid, irrespective of the pressure, but it solvates solid matter as if it were a liquid. A supercritical fluid diffuses freely into and out of adsorbent pores with a minimum of resistance. A major advantage of SFC chromatography is its ability to effect separation of oligomers without derivatization. SFC-GC and flame ionization detection (Chester, 1984) were thought of with the idea of extending the size analytical capability, while simultaneously enhancing resolution and lowering the separation temperature of silyl ethers of corn syrup carbohydrates (Chester and Innis, 1986). Mass spectrometry (MS) is applicable to ionized polysaccharide fragments and molecular ions (Hellerqvist and Sweetman, 1990); these are generated by many techniques (Anderegg, 1990). Food gums display characteristic MS fragmentation patterns (Coates and Wilkins, 1987). Decomposition fragments can be analyzed at selected ion sites. The degree of methylation of pectin has been estimated at mass-to-charge ratios 85 and 96 (Aries et al., 1988). PY-GC alone is sufficient to measure the degree of methylation of pectin (Barford et al., 1986).

[ 46

7. Isolation, Purification, and Characterization

Advances in instrumentation have enabled qualitative and quantitative determinations of polysaccharides in the solid or liquid state by PY-GC-MS. Samples of sols may be injected directly into the gas chromatograph; xerogels may be subjected to PY or inserted directly into the ionization chamber of the mass spectrometer. The mass thermograms and spectrograms permit identification (Donnelly e t a l . , 1980; Morita e t a l . , 1983; Hellerqvist and Sweetman, 1990) through patterns coincident with known molecules: for example, an acetic anhydride pattern indicates the presence of pectin or gum tragacanth (Sjoberg and Pyysalo, 1985); methoxyphenols indicate lignin (Belitz and Grosch, 1987); volatile sulfur compounds are a marker for sulfated polysaccharides and proteins, each distinguishable by its unique pyrolysis products (Merritt and Robertson, 1967; Sjoberg and Pyysalo, 1985). Carbohydrates alter the pattern of accumulation of aliphatic carbonyl and heterocyclic nitrogen compounds that are suggestive of certain proteins and peptide linkages (Merritt and Angelini, 1971).

X. Volume Fraction Using the density equation of dispersed polysaccharides at concentrations obeying Raoult's law, +i may be obtained from V/ vs c i (Walter and Matias, 1989). When V/ vs c i is not linear, +i must be stated at a given c i . Subsequent to 4)i determinations, configurational AS [Eq. (3.22)] were calculated for lowand high-methoxyl pectin. Low-methoxyl pectins were discovered to be less inclined to order themselves in an aqueous medium than high-methoxyl pectins (Walter, 1991).

Xl. Hydrophilicity By the same density approach to +i, the polysaccharide water of hydration was quantified (Walter and Talomie, 1990): H = [13(V~.d~ - 1)]/V~.

(7.13)

H, d e f n e d as the hydrophilicity, is the weight of hydrocolloidal water in grams adsorbed per gram of solute in 10 2 g of dispersion; 13 is the slope of the Vi vs c i graph. Equation (7.13) is based on the proportional nonsolute volume increase when increments of a hydrophilic polysaccharide are added to initially pure water. Not all gums showed the linearity expressed in Eq. (7.13). When there is nonlinearity, e.g., in one sample of konjac flour gum

XIII. Fiber

] 47

dispersions TM (Jacon et a l . , 1993), [3 is the tangent to the curve at a specific c i . Of the conforming polysaccharides tested, the decreasing order of hydrophilicity was CMC > guar gum > methylcellulose > sodium alginate > HM pectin > LM pectin. In the opposite direction, dehydration may also not be linear, but exponentially dependent on time (Lips et a l . , 1988).

XII. Surface A r e a In plotting Eq. (2.15) (the Freundlich equation), a series of flasks is called for that contain different weights of adsorbent (grams; e.g., 0.1-1.0 g) in equal volumes of a test dispersion of solute at the same weight concentration. After an equilibrating period of shaking, the supernatant liquid is filtered, its residual concentration c r is measured, and the filled-sites concentration (amount adsorbed) is obtained by subtraction ( c i - Cr). The graph ( c i - C r ) / g vs ci at constant temperature gives slope 1/k. Alternatively, a constant weight of adsorbent (e.g., 1 g) is placed in equal volumes of dispersion at different c i ; c i - c r is measured and plotted at each c i to a plateau region. Instrumentation is an alternative to chemically measuring surface area. Wheat granule surface area was Coulter-counted (Morrison and Scott, 1986). If a spherical geometry is assumed, Fig. 1 can provide a basis of measuring the total surface area.

Xlll.

Fiber

The naturally occurring, chemically and physically refractory plant polymers of which cellulose is the dominant component are collectively called fiber. Crude fiber (CF) is the residue remaining after a formal acid and alkali scouring. In an official CF assay (AOAC, 1990), the fibrous material is digested with 1.25% H2SO 4 followed by 1.25% NaOH, the digest is filtered, the residue is dried, and then it is ignited. The percentage loss in weight is the CF value--the lowest of all possible fiber values, consisting exclusively of cellulose and lignocellulose. CF was once believed to be the analytical equivalent of human and animal indigestible fiber, and a CF assay was therefore the basic analytical criterion of nutritional value, but because it proved to be inadequate for this purpose, new classifications of food and feed fibers succeeded it. Dietary fiber is defined by nutritionists and physicians as the category of naturally occurring plant components containing soluble and insoluble fiber that increase fecal volume. Soluble fiber consists mainly of pectin and a 18. The manufacturer has since recalled and reengineered konjac flour gum.

] 48

7. Isolation, Purification, and Characterization

fraction of hemicellulose; cellulose, the other gums, lignin, and another fraction of hemicellulose are presumed to be indigestible to humans. Some fiber definitions are based on methodology, e.g., neutral-detergent and acid-detergent fiber. Fiber analyses require initial fat extraction. The highest fiber value obtainable thereafter is the neutral-detergent fiber (NDF) value, consisting of nothing less than dried plant extract from which emulsifiable lipids and a fraction of protein, pectin, and other carbohydrates have been washed away through the detergent action of sodium lauryl sulfate. In a total dietery fiber (TDF) assay, the lipid-extracted substrate is hydrolyzed by cx-amylase (to digest starch), then a protease (to digest protein), then an amyloglucosidase (to hydrolyze branched structures). Insertion of the enzymes in the protocol was an attempt to simulate the action in the human digestive tract (Schaller, 1977). The water-soluble, fibrous fraction (A) is precipitated by alcohol, followed by washing, filtration, and drying of the residue (fraction B). Samples of the dried residue are analyzed for ash by ignition and for protein residue. A + B corrected for residual ash and protein gives the TDF. Replacing the acid, base, andenzyme digestions with refluxing in cetyltrimethylammonium bromide gives acid-detergent fiber (ADF). ADF contains lignin, cellulose, lignocellulose, and insoluble mineral matter, the water-soluble and hemicellulose components having been removed in the supernatant liquid.

XIV.

Pilot Plant Quality

Control

For practical reasons in a food plant where advanced instrumentation is not always available, it is desirable to have recourse to simple techniques to assess quality. Any property of a polysaccharide dispersion can do this if quality factors are adequately referenced.

A. Identification The most elementary, nontechnical method of identifying a polysaccharide is to burn it, observe the yellow-to-brown color, and sniff the perfumelike aroma that should be reminiscent of the aroma of maple syrup, if the test is positive. If the caramelized residue is shaken with egg white and a visible reaction produces an insoluble, pigmented deposit (melanoidin), it is empirical proof that the sample was a polysaccharide.

XIV. Pilot Plant Quality Control

149

Figure 5 Aging of an aqueous, gelatinized starch dispersion showing phase separation with time (10 days).

B. Aging Freshly prepared dispersions are opalescent and light transmittance, already at a minimum, cannot be improved, nor can the dispersions be clarified by filtration; scattering is at a maximum. The properties are reversed in an aging dispersion, as the particles grow larger and fewer and ultimately separate from the solvent (Fig. 5). Aged dispersions are turbid and filterable.

C. Sediment Volume Sediment in an aged dispersion may be collected and measured in a crudely quantitative test. To distinguish between a deflocculated, a flocculated, and an aggregated suspension, a weighed a m o u n t of solid is uniformly suspended in a small quantity of liquid, the suspension is transferred to a graduate cylinder, the volume of sediment during a stated period of time is measured, and the specific sediment volume (milliliters per gram)vs time is plotted. In

150

7. Isolation, Purification, and Characterization

such an experiment, a floc's suspension rate was found to be nearly 3 times that of the aggregate's suspension rate and 10 times that of the deflocculated suspension rate (Ross and Morrison, 1988).

D. Syneresis Alternatively to collecting sediment, liquid may be collected and measured in a conical graduated centrifuge tube from a freshly cut surface of a 200-g sample of jam or jelly; the sample is divided into quadrants on a nylon net. A maximum of 0.5 mL in 2 h is arbitrarily set as an index of good product quality, i.e., a low syneresis tendency (Hercules, Inc., 1985).

E. Consistometry In Bostwick consistometry, a constant volume of a high-solids suspension (e.g., tomato ketchup, applesauce) is permitted to flow unidirectionally for 30 s along a graduated path in a rectangular, stainless steel trough. Quality grades are preset to correspond with the distance (in centimeters) traversed by the forward edge of the suspension. A US Grade A tomato ketchup requires a distance of 9 cm in 30 s at 20~

F. Texture The most generalized property of a polysaccharide semisolid dispersion is texture, for which there are any number of definitions (Bourne, 1982), each nevertheless suggesting a physiological response to physical stimuli (size, shape, flow, hardness, etc.). Objectively, this elusive property is measured as the force necessary to compress or puncture the test object. The TA-XT24 texture analyzer (Texture Technologies Corp., Scarsdale, NY) is a device mounted with various mechanical probes, calibrated with weights, and preset to penetrate the test object at variable speeds to variable depths. The magnitude of the force and the shape of the force-time profile (Rao et al., 1989) provide information about fracturability, cohesiveness, springiness, and gumminess of the test material. The Instron Universal Testing machine (Instron Engineering Corporation, Canton, MA) and the Voland texture analyzer (Voland Corporation, Hawthorne, NY) are similar instruments.

XV. Polysaccharide Theta Conditions

1 51

XV. Polysaccharide Theta Conditions Temperature and solvent define the 0 state. A polysaccharide lower Tc is in the range of refrigerated temperatures where solute-solvent interaction yields to solute-solute interaction" defining the 0 temperature is thus restricted to 25-28~ Figure 6 suggests that the viscosity of ionic polysaccharides in dilute d-tartaric acid (TA) and of nonionic polysaccharides in water (c i = 0.05-0.07%) are in the same general "qi - ci orbit at 28~ A sample of CMC (0.05 g) was dispersed in 80-mL water in 100-mL beakers to which TA was afterward added to different molarities; TA supplied the H + counterion intrinsic to an ionic polysaccharide and the nonintrusive tartrate ion. The solutions were transferred to 100-mL Erlenmeyer flasks and brought to volume with water prior to dilution viscometry. Judging from Fig. 7, a molar concentration of TA approximating 0.35 ensures an 'rlsp/Ci m i n i m u m in a dilute CMC dispersion (c i = 0.05-0.07%).

14.0 12.0 10.0

""---x

] ~ s ; c 8.0

4

6.0 4.0

2.0-

0

O. 1

i

0.02

I

0.03

I

0.04

I

0.05

I

0.06

I

0.07

I

0.08

C

Figure 6 Viscosity profile of gellan in water (1), gellan in 0.04-M tartaric acid (2), locust bean (3) and methylcellulose (4) in water, and CMC in 0.04-M tartaric acid (5).

152

7. Isolation, Purification, and Characterization 0.0

8.0

7.0

6.0

Tlsp/e 5.0

4.0

3.0

2.0

1.0

o

!

0.05

i

o.,o

!

o.ls

o. s

o.;s

o.;,s

M

Figure 7 Viscosity profile at 28~ tartaric acid.

of 0.05% CMC in different molal concentrations ( M ) of

From Alfrey's (1947) analysis, the region of the negative slope of polyanions before the minimum viscosity function (gellan in Fig. 6 and CMC in Fig. 7) is a region of declining solute-solvent interaction, followed by an increasing solute-solute affinity; the latter continues through to a positive slope after the minimum. On the strength Of the results in Fig. 7, a second series of dispersions was similarly made with different concentrations of CMC in 0.35-M tartaric acid (Fig. 8). Flow times were 57 s for 0.05% CMC in 0.35-M tartaric acid and 76-96 s for the range of CMC dilutions. There was no minimum in evidence at dilute CMC concentrations, and as seen in Fig. 8, "rlsp/Ci increased directly albeit slowly with c i, which is presumptive evidence that solute-solute

XV.

Polysaccharide

Theta

153

Conditions

10.0 9.0 8.07.0 p/

TIs c

6.05.04.03.02.01.00

'

I 1.2

F i g u r e 8 Viscosity profile at 28~ tartaric acid solution.

'

i

I 2.4

'

(c x 10 "3)

I 3.6

'

I 4.8

I 6.0

of aqueous CMC at different concentrations (c) in 0.35 M

interaction exists in dilute (CMC) dispersions. However, the viscosity n u m b e r decrease [Xlsp/C (9.5-7.7)] was merely 20% for a quadrupling of the concentration (c = 4.8-1.2 X 10-3). Given interaction on either side of the minimum, the 0 state is arguably unachievable with polysaccharides, but may be approximated. The most that can be said of these data is that the 0 condition is approached at concentrations in the third decimal. Neutral polysaccharides displayed similar relationships, omitting electroviscosity. The effect of 40~ on CMC dispersions was studied and found to be quite similar to the effect at 28~ (Fig. 9). The 40~ line gave a correlation coefficient of 0.81. When the point at 4.0 X 1 0 - 4 w a s removed, the correlation coefficients rose to 0.95, suggesting marginal sensitivity at that level of dilution. Figure 10 shows that the effect of a hydrophobic solution environment was identical on a dilute ionic (CMC) and a neutral polysaccharide at the same dilute concentration. In each instance, the slope of the line approximated 0. Recognizing the dilution effect on polyanion viscosity, it may be argued that ethanol is as efficacious as electrolytes in inducing not only nonionic behavior in dilute polysaccharide polyanions, but in controlling solute-solvent interaction in preparation for M and Rg measurements.

7. Isolation, P u r i f i c a t i o n , and C h a r a c t e r i z a t i o n

154

1.00 '

0.10

TI,p .010

X

2

9

.001

n (c x 10 -n) F i g u r e 9 The effect of temperature (28~ line 1 and dots, and 40~ line 2 and multiplication signs) on the specific viscosity (%p) of a 0.05-wt/vol% CMC dispersion.

20.0

18.0 P

1

9

9

16.0

TIsJc

; 4.0

.'

.

"-

2

-

1 KONJAC 2 CMC

3.0

f

o

I

,

.o

2'.o

3'.o

4'.0

s'.o

6'.0

% Ethanol Figure I0 The effect of ethanol (vol/vol%) on the viscosity number (Tlsp/C) of a 0.05-wt/vol% konjac (1) and CMC (2) dispersion.

XVII. Summary

155

XVI. Blending The blending chart in Fig. 1 in Chapter 5 shows an exponential dependence of solution viscosity on concentration, within the range of maximum measurable concentration (100%). Rheometry permits higher c i than does viscometry. As a prerequisite to construction of a linear polysaccharide blending chart, the linear range of "qsp/Ci vs ci should first be established, because it is only then that the viscosity of the blend will be equal to the sum of the viscosities of the components. Each ordinate in Fig. 1 in Chapter 5 is labeled with the maximum c i of the two polysaccharides of interest or its equivalent as 100%.

XVII. Summary Many properties in common often make polysaccharides analytically indistinguishable from each other, except for differences occasioned by ionic and nonionic composition. However, subtle differences occasionally exist, so that a specific test may be designed for a specific molecule. Polysaccharides almost always require tedious isolation and purification techniques to insure a high degree of relative purity before characterizing methodologies are applied. The uniqueness of the hydrocolloidal response to ambient stimuli enables them to be characterized nondestructively in their dispersion surroundings. Concentration dependence, solvent conditions, thermal effects, transitions, etc., are defining variables of their functional utility. Reactive chemicals, heat, and critical time-temperature integrals partially or totally decompose them. The decomposition products are useful as flavorants, colorants, and chemical markers. Especially in the presence of nitrogen compounds, pyrolysis yields the universally pleasant aromas of bread, cakes, and nuts. Carbohydrate and carbohydrate-protein thermolytic reactions are the cause of the yellow-to-brown coloration observed in many cooked carbohydrate foods. The size of a polysaccharide molecule is an indeterminate property: molecular weight, for example, depends on the analytical method. Electroviscosity and solute-solvent and solute-solute interactions affect the volume-gram relationship (density) and conformational dimensions. The degree of polymerization is not affected and is therefore a constant, reliable indicator of the molar mass. Elementary quality control tests involving a visible polysaccharide property or response can suffice in quality control laboratories for the lack of advanced instrumentation.

This Page Intentionally Left Blank

CHAPTER 8

Classifications I. I n t r o d u c t i o n Polysaccharides are broadly divisible into two main groupsmionic and nonionic. Otherwise they possess many properties in common that transcend empirical class boundaries. They are systematically classified according to origin, isolation method, function, texture, thermoreversibility, and gelling time (Trudso, 1989) based on the classifiers' interest: for example, the science nomenclator organizes polysaccharides by chemistry, but to the food technologist, a class of nongelling polysaccharides suggests that an aqueous dispersion of any member of the class does not solidify under food processing and preparation conditions, where functionality is the primary interest. By polysaccharide functionality is meant the ability of a polysaccharide to impart properties to or assist in improving edible matter from the standpoint of human nutritional and food-processing quality. Tables I and II are an indication of the uses and applicable concentration ranges of common food polysaccharides.

II. C h e m i c a l

Classification

Chemical classification of polysaccharides is the least ambiguous system of grouping these macromolecules. Polysaccharides of different origins can have similar structures, as Kravtchenko et al. (1992) discovered in lemon and apple pectin. Likewise, polysaccharides of the same origin can have different structures, as for example, the concentration of pyruvate and acetyl substituents in xanthan, depending on incubation conditions (Pettitt, 1982).

157

TABLE I Properties and Uses of Polysaccharidesin Food' ~

~~

Property

VI

W

~~

~

~

Low methoxyl pectin

High methoxyl pectin

Iota carrageenan

Kappa carrageenan

Lambda carrageenan

Solubility in water

Sol. cold and hot

Sol. cold and hot

Sol. above 70°C, Na' and NH: Sol. cold

Sol above 70°C. Na+ and NH:. Sol. cold. K+ and Ca2+ swell cold to thixotropic dispersions

Sol. cold and hot

Solubility in milk

Sol. cold and hot

Sol. hot

Sol. above 70°C

Sol. above 70°C

Sol. hot. Swells cold

Solubility in salt solutions

Insoluble

Insoluble

Insoluble

Sol. hot

Sol. hot

Solubility in sugar solutions

Sol. hot

Sol. hot

Sol. hot

Insoluble

Sol. hot

Solubility in ethanol

Insol. above 20%

Insol. above 20%

Insol. above 20%

Insol. above 20%

Insol. above 20%

Other factors influencing solubility

Increases with decreasing M W , increasing randomness of COOH, decreasing sugar and Ca2+

Increases with decreasing sugar and Ca"

Increases with decreasing Na+, K' and, Ca2

Increases with decreasing Na', K', and Ca2'

Increases with decreasing Na+, K+, and Ca"

Solution viscosity

Low

Low

Low

Medium

High

Optimum pH range

2.5-4.0 pK, 3.3

2.5-5.5

4-10

4-10

4-10

Optimum soluble solids range

55-80%

30-80%

0-40%

0-20%

0-80%

pH below 4 and sol. solids 55-80%

Presence of Ca2+ 10-70 mg/g pectin. Temp. below setting temp.

Presence of K+, Na' or Ca2+.Temp. below setting temp.

Presence of K', Na+ or Ca2+.Temp. below setting temp.

Nongelling

Cohesive no syneresis, thermoirreversible

Cohesive to brittle. Brittleness increases with increasing Ca2+ and decreasing sugar. Thermoreversible

Strong, brittle. Brittleness increases with increasing K+ and Ca2+ and decreasing LBG. Thermoreversible

Soft, cohesive, thixotropic. Thermoreversible Thixotropy is lost with addition of LBG

Nongelling

Setting temp.

Increases with increasing DE, decreasing pH and increasing sugar

Increases with decreasing DE, increasing DA, increasing Ca2' and increasing sugar

Increases with increasing K+, Na+, Ca", and sugar

Increases with increasing K+, Na', Ca2+, sugar, and LBG

Nongelling

Gel strength

Increases with increasing concentration and M w Precipitation

Increases with increasing concentration and Ca2 Gelation

Increases with increasing concentration, K + , Ca2', and LBG Ionic interaction. Increased gel strength

Increases with increasing concentration, K f , Na', and Ca2+ Ionic interaction. Increased gel strength

Nongelling

Adsorption to casein particles below pH 4.2. Adsorption to soy protein particles below pH 4.8.

None

Precipitation below iso-pH

Precipitation below isepH

Precipitation below iso-pH

Gelation conditions

Gel characteristics: Texture

-cn r9

Effect on milk at neutral pH Effect on milk and other proteins at acid pH

+

Ionic interaction. Increased viscosity

continued

TABLE I continued

Incompatibility

Low methoxyl pectin

Kappa carrageenan

Water soluble alcohols, ketones, heavy metals, quaternary detergents, cationic macromolecules

Water soluble alcohols, ketones

Water soluble alcohols, ketones, quaternary detergents, cationic macromolecules

Property

-m 0

Iota carrageenan

High methoxyl pectin

Property

Guar gum

Xanthan gum

Water soluble alcohols, ketones, quaternary detergents, cationic macromolecules

Lambda carrageenan Water soluble alcohols, ketones, quaternary detergents, cationic macromolecules

Gelatin

Gum arabic

~~

Solubility in water

Sol. cold and hot

Sol. cold and hot

Sol. above 40°C

Sol. cold and hot

Solubility in milk

Sol. cold and hot

Sol. cold and hot

Sol. above 40°C

Sol. cold and hot

Solubility in salt solutions

Sol. cold and hot

Sol. cold and hot

Sol. above 4OoC

Sol. cold and hot

Solubility in sugar solutions

Sol. cold and hot

Sol. cold and hot

Sol. above 40°C

Sol. hot

Solubility in ethanol

Insol. above 20%

Insol. above 50%

Insol. above 20%

Insol. above 60%

Increases with decreasing Mw

Increases with increasing pH up to 6

Other factors influencing solubility Solution viscosity

High cold. Low hot

High below 100°C

Low

Low

Optimum pH range

4-10

1-13

4.5-10 iso-pH 4.8-5.2 (limed) iso-pH 6.0-9.5 (acid)

2-10

Optimum soluble solids range

0-80%

0.80%

0.80%

0.80%

Gelation conditions

Nongelling

Presence of LBG, tara gum, cassia gum. Temp. below setting temp.

Temp. below setting temp.

Nongelling

Gel characteristics: Texture

Nongelling

Cohesive, gummy, thermoreversible. Guar makes texture of xanthan/LBG gel more brittle

Soft to strong, cohesive, gummy. Thermoreversible

Nongelling

Setting temp.

Nongelling

Constant

Increases with increasing MW and maturing temperature

Nongelling

Gel strength

Nongelling

Increasing with increasing concentration

Increases with increasing concentration and decreasing salt

Nongelling

Effect on milk at neutral pH

Separation

None

None

None

Effect on milk and other proteins at acid pH

None

Precipitation below iso-pH

None

None

Incompatibility

Water soluble alcohols, ketones

Water soluble alcohols, ketones, gum arabic below pH 5

Water soluble alcohols, ketones, anionic macromolecules below iso-pH, gum arabic below iso-pH

Water soluble alcohols, ketones, alg., gelatin, xanthan gum

continued

TABLE I continued

Property

01

Agar-agar

Alginate

K', Na', NH: Sol. cold and hot Ca2+ Insol. at neutral pH Insol. Na' swells in boiling milk. Sol. with sequestering agents

Propylene glycol alg.

Cellulose gum

Locust bean gum

Sol. cold and hot

Sol. cold and hot

Sol. above 85°C

Sol. cold and hot

Insoluble

Sol. above 85°C

Solubilityin water

Sol. above 90°C

Solubility in milk

Sol. above 90°C

Solubility in salt solutions

Sol. above 90°C

Insoluble

Inso1u b 1e

Insol. High DS types sol.

Sol. above 85°C

Solubility in sugar solutions

Sol. above 90°C

Sol. hot

Sol. cold and hot

Sol. cold and hot

Sol. above 85°C

Solubility in ethanol

Insol. above 20%

Insol. above 40%

Insol. above 40%

Insol. above 30%

Insol. above 20%

Other factors influencing solubility

Increases with decreasing COOH, increasing pH, decreasing Ca2'

Increases with decreasing M W , increasing pH, increasing divalent cations

Solution viscosity

Low

Low above pH 5.5 High helow pH 5.5

High

High

High up to 85°C

Optimum pH range

2.5-10

2.8-10, pK, 3.4-4.4

2.8-10

3-10 pK, 4.2-4.4

4-10

Optimum soluble solids range

0-80%

0-80%

0-80%

0-80%

0-80%

Gelation conditions

Temperature below 32-39°C

pH below 4 or presence of Ca2+ 20-70 mg/g a k .

Nongelling

Nongelling (gelation may occur with trivalent cations)

Nongelling

Strong, brittle. Thermoreversible. Brittleness increases with increasing sugar

Acid gels soft, cohesive and thixotropic. Calcium gels strong, brittle, Thermo-irreversible

Nongelling

Nongelling

Nongelling

Constant Increases with increasing concentration, increasing sugar and increasing pH

Nonexistent Increases with increasing concentration, Ca2+ and decreasing pH down to 3.6

Nongelling Nongelling

Nongelling Nongelling

Nongelling Nongelling

Effect on milk at neutral pH

None

None. Insoluble

None

Precipitation

Separation

Effect on milk and other proteins at acid pH

None

None

None

Adsorption to casein particles below pH 4.6. Adsorption to soy protein particles below pH 5.0

None

Incompatibility

Water soluble alcohols, ketones

Water soluble alcohols, ketones, milk, gum arabic

Water soluble alcohols, ketones

Water soluble alcohols, ketones, quaternary detergents, cationic macromolecules

Water soluble alcohols, ketones

Gel characteristics: Texture

-

Setting temp. Gel strength

OI W

“From Trudso, 1989. Reprinted with permission.

164

8. Classifications

TABLE II Function and Concentration Ranges of Polysaccarides in Food.

Hydrocolloid Pectin

Carrageenan

Agar-agar

Alginate

CMC

LBG

Guar gum

Xanthan gum

Food Jams, jellies, preserves Bakery fillings, glazings Fruit preparations Fruit beverages, sauces Confectionery Dairy products Ice cream Chocolate milk Flans and puddings Liquid coffee whitener Low calorie jams Dessert gels Tart glazing Meat products Pimiento paste Salad dressing Icings Confectionery Meat products Dairy products Ice cream Icings Toppings Salad dressings Beer Fruit drinks Restructured foods Simulated fruit Ice cream Ripples Sour milk Cake mixes Icings Batters Dry-mix beverages Syrups Ice cream Cream chesse Dessert gels Ice cream Cottage cheese Processed cheese Cake mixes Bakery jellies Fruit drinks Cream cheese Baked goods Dressings

Function

Concentration (%)

Gelation, thickening Gelation, thickening Thickening, stabilization Thickening, stabilization Gelation, thickening Stabilization, gelation Stabilization Stabilization Gelation, thickening Thickening Gelation, thickening Gelation Gelation Gelation, water binding Gelation Stabilization Gelation Gelation Gelauon Gelauon Stabilization Gelation Gelauon Stabilization Stabilization Stabilization Gelation Gelation Stabilization, thickening Thickening Stabilization Moisture retention Thickening, water binding Thickening, stabilization Thickening Thickening Stabilization Thickening, moisture control Gelation, water retention (together with carrageenan) Stabilization Thickening Moisture retention Thickening Gelation, thickening Pulp suspension Gelation Moisture retention Stabilization

0.1-1.0 0.5-1.5 0.1-1.0 0.01-0.5 O.5-2.5 0.1-1.0 0.01-0.03 0.01-0.03 0.1-0.5 0.1-0.2 0.8-1.2 0.6-1.1 0.8-1.0 0.3-0.5 1.5-3.0 0.3-0.6 0.1-0.3 0.3-1.8 O.5-2.O 0.05-0.9 0.1-0.5 0.1-0.5 0.3-0.5 O.2-O.5 0.O04-0.0O8 0.1-0.3 0.6-1.0 0.8-1.0 0.1-0.3 0.1-0.4 0.1-0.2 0.2-0.4 0.1-0.2 0.2-0.4 0.1-0.3 0.2-0.6 0.2-0.3 0.3-0.6 0.3-0.6 0.2-0.3 0.3-0.6 0.2-0.4 0.1-0.2 0.1-0.3 0.02-0.06 0.1-0.2 0.1-0.2 0.2-0.3

II. Chemical Classification

165

T A B L E II continued

Hydrocolloid Gelatin

Gelatin

Gum arabic

Food

Function

Concentration (%)

Yogurt Dessert gels Confectionery Meat products Mousses Minarine Flavor fixation

Gelation Gelation Gelation Gelation Stabilization Stabilization Encapsulation

0.3-1.0 4-6 3-10 1-5 1-3 1-3 80-90

Confectionery Flavor emulsions

Stabilization, gelation Stabilization,emulsification

10-60 10-30

aFrom Trudso, 1989. Reprinted with permission.

A. eL-D-Glucans

This group of nonreducing glucose polysaccharides contains a preponderance of 1,4-OL-D-glycopyranosyl linkages. The most ubiquitous representative is starch. 1. Starch

Native starch (Whistler and Smart, 1953; Biliarderis, 1992) is deposited in granules in plant tissues and is constructed of amylose (1,4-a-D bonding) and amylopectin (1,4-a-D bonding in the primary structure and 1,6-a-D bonding at branch points). Potatoes and legumes are among the highest natural accumulatorsm34-70% in peas (Zuber, 1965), 20-36% in ordinary corn, and 16-17% in rice. Granule sizes and morphologies are typical of the genus, from irregular 10-25-~m-diameter polyhedrons in corn to uniform polyhedrons in rice (3-8 p~m), and spheroids in potatoes (15-100 ~m; 33 ~zm, average). Other geometries encountered are ellipsoids, polygons, platelets, and tubules (Roller, 1996). The proportion of amylose to amylopectin varies with the plant source; normally it is about 1:3. Waxy maize and waxy rice contain little or no amylose: waxy maize starch cooks to a clear, stable, nongelling paste, whereas regular corn starch cooks to a cloudy, retrograding gel. Amylomaize is starch from corn that has been bred to contain as much as 70% amylose. The optical anisotropy resulting from morphology creates unique polarization crosses that are generally distinguishable by phase microscopy. Amylose was cited as an example of a non-free-draining random coil (Ring and Whittam, 1991). Starch is considered to be a neutral molecule, but potato starch has a low degree of phosphation and is thereby endowed with weak polyelectrolytic character. Starch phosphate esters reside principally in the amylopectin fraction.

166

8. Classifications

The mechanical integrity of starch granules is inversely related to size" small granules are more resistant to dry heat than are large granules. During extrusion, corn and potato starch, containing the largest granules, disintegrate more than did rice starch (Mayer, 1993). Given the numerous plant origins and macromolecular heterogeneities (composition, granule size, shape, etc.), starch gelatinizes, not at a single temperature, but over a narrow range of temperatures, commonly 50-80~ whose midpoint is cited as Tg z . Each generic starch has a characteristic Tg z . Concentration, the amylose-amylopectin ratio, the DP, pH, electrolytes, and cosolutes (sugars) are other influences on the exact location of Tgz . The smaller the DP, the lower is Tgz ; this is explained by the greater kinetic activity of smaller particles. Starch granules remain refractory assemblies, once the temperature is maintained below Tgz . Suspended wheat granules held at Tgz for 72 h increased in size, porosity, and adsorbent properties while retaining their anisotropy; subsequently they gave a narrower Tg~ range and higher Tg~ than the control granules (Gough and Pybus, 1971). Of the common starches, high-amylose corn and rice starch have the highest Tg~ (74.5 and 80~ respectively); barley and tapioca have the lowest (57~ wheat (61~ and potato (63~ are intermediate (Glicksman, 1969). For the same intrinsic reason of a variable Tg~, starch sols show widely differing gelling concentrations in water. Potato starch, typically containing the largest granules, gels at 20% concentration in water; waxy corn starch gels at approximately 30% concentration (Osman, 1967); corn starch gels at approximately 5% concentration. Gelatinization is reversible in the first stage (hydration). As the event progresses, there is a gradual loss of polarization and an irreversible loss of birefringence. Gelatinized starch is m o r e susceptible to acid, heat, and enzymes than granular starch. Cooking makes starchy foods digestible as a result of gelatinization. There is some debate about the role of amylose in crystallite formation, because of its great mobility (Rutenberg, 1980). Contrary to the expected correlation between parallelism and crystallinity, Banks and Greenwood (1975) and Cairns et al. (1991) found evidence to implicate principally amylopectin, rather than amylose. Crystals did remain after amylose was leached from the granules (Montgomery and Senti, 1958). Although waxy starches are highly branched, they show signs of localized organization where there is a high concentration of closely packed, linear segments. The rate of retrogradation is time-dependent and inversely temperaturedependent. The opacity developed in stored starch sols, the staling of bread, etc., are retrogradation phenomena initiated by aging, progressing at a faster rate at a lower temperature. Deterioration is repressed by sodium chloride (Rutenberg, 1980) and sugar (Miles et al., 1985), and is customarily delayed in starch dry mixes by diluents (sugar), anticaking additives (calcium citrate, aluminum silicate, acid phosphate), and crystallization inhibitors (ungelatinized starch, protein, hemicellulose, invert sugar, and other hydrocol-

II. Chemical Classification

167

loids). Retrograded starch will partly or wholly redisperse by adequate heating in a moist atmosphere; the softening of bread by microwave reheating is based on this principle. An amylogram of granulated starch shows viscosity passing through a maximum (the peak viscosity) at Tgz and falling precipitously, if the temperature is raised further; this response is also exhibited during shearing. The collapse of the granule and tertiary structures, a result of too much mixing, explains the concave upper surface seen in failed breads and cakes. A superior quality of breads, cakes, and pastries contains a fair percentage of intact granules that helps to strengthen the batter or dough against thermal collapse. In a subsequently cooled starch paste, a weak structure is reformed and a secondary viscosity maximum called setback is observed. The higher the amylose content of starch, the higher is the setback on the numerical scale of the amylograph. In the manufacture of "instant" food items, starch is precooked in a moist atmosphere, then dehydrated and formulated with compatible ingredients into dry mixes. At serving time, water or milk is added, with or without mild heating, and the instantly rehydrated product is ready to be consumed. The broken-curve heating profile of intermediate-moisture, thixotropic starchy products pictorializes gelatinization and solid-liquid (gel-sol) transitions. At high starch concentrations, the initial suspension heats by convection, because of the high heat conductivity of water. Subsequently it heats by conduction, as the gelatinized starch is released into the aqueous medium, thickening it to a gellike consistency. An initially solid mass may revert to a liquid mass; then heat convection and conduction are reversed. Qu and Wang (1991b) ascribed sensorial appeal to the "melting" of starch granules and the ratio of gelatinized to melted starch. Other factors to consider are pH, oral stress, temperature, and concentration (Olkku, 1978). For mechanical stability against hydration and swelling, starch is chemically crosslinked (by adipic acid-acetic anhydride, phosphate, etc.) below Tgz, so that the physical integrity of the granules is not impaired by gelatinization. Weakly crosslinked starches (DS < 0.1) are acid-stable and thus find occasional application in recipes containing vinegar. The many industrial functions of crosslinked starch include use as an adhesive.

2. Modified Starches Substituent groups have been incorporated into dry or suspended starch, mainly to improve its properties and to stabilize it against retrogradation: dispersion clarity is enhanced by substitution (Wurzburg, 1995). Acetylated starch is more mechanically stable, has a lower cooking (swelling and gelatinization) temperature, and is less prone to gel than the parent starch. Phosphated starch has the advantages of acetylated starch and the extra advantage, in some instances, of high viscosity. The carboxyl-

] 68

8. Classifications

ated, sulfated, and phosphated starches suffer from electrolyte sensitivity. Hydroxyethylstarch and hydroxypropylstarch are freeze-thaw, acid-stable ethers more indifferent to long heating times and high temperatures than other derivatized food starches. Cationic starches are the product of chemical derivatizations with nitrogen-, sulfur-, and phosphorus-containing reactants; they become positively charged and therefore useful as sequestrants of organic and inorganic anions. Many other substituted starches have applicability in food and nonfood industries (Wurzburg, 1986). Actual and potential uses include sizing textiles and paper and as adhesives soluble in organic solvent. 3. Dextran

Dextran is a structurally heterogeneous glucan containing linear and branched sequences with a preponderance of 1,6-oL-D (isomaltose) and a smaller percentage of 1,2-oL, 1,3-~, and 1,4-a-D linkages. This heterogeneity results in easy dispersion in hot and cold water. Insolubility is associated with 1,3-or linkages (Sidebotham, 1974). The most important application of dextran has been in medicine, where the 104-105-Da homologs are used as a blood-plasma substitute. The main industrial use is as a crosslinked gel bed, sulfated for use in ion exchange. Dextran has emulsion-stabilizing properties at low ionic strength, and creaming properties at high ionic strength (Dickinson et al., 1989); it can be a problem in sugar factories where spontaneous fermentation can plug filters and hinder crystallization. 4. Pullulan

Pullulan is a linear, water-soluble polysaccharide containing repeating trimers of 1,4-ot-D-glucopyranose and 1,6-a-D-glucopyranose in a 2:1 ratio. Its films are touted for their similarity to polyethylene (Glicksman, 1982).

B. 13-D-Glycans Cellulose (Ott, 1943; Corbett, 1963; Green, 1963) is the most dominant biopolymer in this class. 1. Cellulose

Cellulose is the 1,4-~-D anomer of starch. It has a repeating cellobiose dimer that exists in vivo as closely packed 2-30-nm-long microfibrils embedded in a noncellulose matrix. The microfibrils are thoroughly hydrated in succulent fruits and vegetables in which moisture can be as high as 95%.

II. Chemical Classification

169

Once dehydrated, the microfibrils are practically without functionality in ordinary food processing and preparation operations, because the inert microcrystallites are difficult for water to penetrate. The polymorphs, cellulose I and II (Blackwell, 1982; Coffey et al., 1995), are differentiated by their molecular orientation, hydrogen-bonding patterns, and unit-cell structure. Cellulose I is the natural orientation; cellulose II results from NaOH treatment under tension of cellulose I with 18-45% alkali (mercerization). The I-II transition is irreversible. Mercerization strengthens the fibers and improves their lustre and affinity for dyes (Sisson, 1943). Sewing thread was relatively pure mercerized cotton until the advent of synthetic polymer fibers. Cellulose is designated or, [3, and ~/ on the basis of alkali solubility. a-Cellulose is that fraction not removed by treatment with 17.5% NaOH at 20~ the 17.5% NaOH-soluble fraction contains [3- and ~/-cellulose. The subfraction precipitating after acidification of the alkaline liquid phase is [3-cellulose; ~/-cellulose is the acid- and base-soluble subfraction remaining dispersed, or-Cellulose contains the highest DP. Native cellulose is engineered into a variety of utilitarian forms (Fig. 1). Cotton is the cellulose fibers harvested from around the seeds of Gossypium sp.: it experiences an infinite number of applications, chiefly in the textiles and medical industries. Solka-floc is purified, shredded, fluffed wood cellulose noted for its absorbency. Paper is the final product of wood cellulose that is mechanically disintegrated to free the fibers, agitated to intertwine them and strengthen the fiber network, and pressed to smooth the network surface. Mechanical pulp may be chemically treated further to remove impurities before formulation to desired specifications with additives. By steeping shredded-wood alkali-cellulose in CS 2 (xanthation) and extruding the viscous fluid (viscose) three dimensionally as fibrils (rayon) or two dimensionally as sheets (cellophane) into an acid bath, a translucent cellulose gel is regenerated. This regenerated cellulose, per se, when dry, is both grease- and oil-proof and, for all practical purposes, gas impermeable (Flexel, 1989), but it does not provide moisture protection. Plasticizers, sealers, sizers, and other chemicals are added to viscose to confer miscellaneous properties on cellophane. For some uses, nitrocellulose and polyvinylidene chloride are layered onto the surface of the sheets to improve their poor moisture barrier property (Reiter, 1986). Figure l d is cellophane coated with saran (a vinylidine copolymer) for moisture and chemical resistance and thermoplasticity (heat sealability). An essential difference between cellophane and pulp is the absence of crystalline sites in the cellophane; they are removed by alkalization and xanthation, which consequently intensifies the transmitted light vis-~t-vis the scattered light. A certain high quality of paper (bond) scatters light profusely, due to the inclusion of titanium dioxide as a coating and filler additive. Cellulon is the trade name of a cellulose newly developed from bacterial fermentation. It is reported to have unique properties as a result of its small

170

8. Classifications

Figure I

Utilitarianforms of cellulose.

fiber size (0.1 p,m diameter), which enables it to expose 200 times more surface area than other cellulose fibers. Cellulon is claimed to have great binding, shear-thickening, and coating power at very low concentrations, and to be synergistic with other thickeners (Krieger, 1990). The amorphous regions of cellulose are less chemically and physically reactive than their starch counterparts, but may nevertheless be hydrolyzed to a "level-off' or limiting concentration of 40-70% of intact, pure, rodlike crystallites several hundred angstroms long and less than 100 A wide (Tanford, 1961). Limit cellulose facilitates the prerequisite rodlike ordering for optical anisotropy and precipitation. Limit cellulose is further partially degraded by mechanical attrition to a microcrystalline DP of 200-300 (Battista and Smith, 1962), sold under the brand name Avicel (FMC, 1993). Avicel in aqueous suspension exhibits weak hydrophilic character and, by stiffening films and foams, it is a strong transparent barrier to lipid migration. It stabilizes foams by organizing itself in tandem into gellike, fibrillar, heat-stable structures around bubbles of air (FMC, 1993). Thermophysical and thermochemical refractoriness make Avicel a premier foam stabilizer of retorted polysaccharide foodstuffs. The life of an Avicel suspension can be extended by coprecipitating the rodlike structures with a protective colloid after trituration. Avicel-RC 19 is limit cellulose that has been physically modified by coprecipitation with CMC to facilite dispersibility. Avicel-RC water suspensions simulate the properties of a hydrosol. At low aqueous concentrations, the apparently hydrated crystallites assemble into a thixotropic, heat- and acid-stable structure whose viscosity depends directly on pH to about pH 10, whereupon it declines precipitously. The suspension coalesces at low pH. The addition of salt after mixing increases viscosity above what it would be if the salt were added at the time of mixing or shearing. 19. RC is an abbreviation for regenerated cellulose.

II. Chemical Classification

1 71

A microdefibrillated suspension of cellulose in water has been reported to have an indefinite shelf-life (Weibel, 1994).

2. Cellulose Derivatives

Controlled derivatization to various DS as low as 0.05 causes cellulose to lose its crystalline habit. The family of these derivatives is known collectively as cellulose gums. The properties of cellulose gums are a function of the nature, quantity, and distribution of the substituent(s); their rheology is affected much less by ester and ether content than by molecular weight (Krumel and Sarkar, 1975). Most cellulose derivatives are acid-stable over the normal range of food acidities and, unlike other polysaccharides, a number of them gel when heated, and return to the sol state when cooled: the gelation mechanism is crystallization (Sarkar, 1979). Alkyl and hydroxyalkyl substituents cause cellulose derivatives to tolerate high concentrations of ethanol (20%). Given the inherent stiffness of the cellulose primary structure, these derivatives tend to behave as liquid crystals (Stannett, 1989), i.e., they are ordered at rest and disordered when disturbed. Methylcellulose (Dow Chemical Co., 1990) is a water-soluble, thermostable ether that gels reversibly upon heating at 40-50~ Sucrose lowers Tgel. The gels are prone to hysteresis (Grover, 1993). This biopolymer is an excellent foam stabilizer, as demonstrated in Fig. 1 in Chapter 2 by the visible retention of air bubbles at the solid-liquid interface. Hydroxypropylcellulose is also a water-soluble ether that is acid-stable, strongly lipophilic, and has gelling characteristics identical to those of methylcellulose. Gels from the lower DP homologs are harder, more rigid, and more dimensionally stable than gels from higher DP homologs (Desmarais and Wint, 1993). Food-grade CMC is a cellulose carboxylic acid ether with an optimum DS = 0.4-0.7. The higher the DS within this range, the more hydrophilic is the polyanion. Uniformity of substitution makes CMC more compatible with dissolved salts and less inclined to thixotropy than uneven distribution (Feddersen and Thorp, 1993). This gum does not precipitate from a 50% ethanol solution. Below approximately pH 4 in water, the polyanions revert to the un-ionized, water-insoluble acid. CMC viscosity-hysteresis has already been described (Fig. 2 in Chapter 3). CMC dispersions and films have the extra advantage of transparency relative to many other polysaccharide dispersions. The films are resistant to oils, grease, and organic solvents (Hercules, Inc., 1980). Chitin is the naturally occurring, insoluble acetylamino derivative of cellulose yielding the polycation chitosan when deacetylated (with concentrated NaOH and heat). Chitosan is water-soluble in proportion to the degree of deacetylation.

172

8. Classifications

3. Curdlan

Curdlan (Kimura et al., 1973; Sandford, 1979; Nakao et al., 1991; Harada et al., 1993) is a 1,3-[3-D-glucan whose cloudy aqueous suspensions develop a soft, thermosetting, freeze-thaw-stable, translucent gel when heated above 80~ Gelation may be induced by the action of Ca 2+ in weakly alkaline dispersions. Curdlan gels are hard and brittle in the acidic pH range, but soft and elastic in the alkaline pH range. Gel strength increases with increasing temperature and duration of heating. Curdlan gel properties are exploited in the fabrication of a wide assortment of textured foods, with the proviso that they be protected from premature exposure to heat, because of the flow irreversibility of the thermoset. Curdlan xerogels swell in cold water but do not disintegrate.

4. Glycomannans

This group of polysaccharides is represented by locust bean and guar gums (1,4-[3-D-mannan with 1,6-~-D-galactose substituents) and konjac gum (1,4-[3-D-glucomannan). These gums are remarkable for the synergistic effect they have on other polysaccharides and proteins and their tendency to self-associate. Locust bean gum is also called carob gum. Guar gum (Hui and Neukom, 1964; Sprenger, 1990) is a shear-stable and hence cold-water-dispersible 1,4-[3-D-mannose in linear configuration, with 1,6-1inked ot-D-galactosyl substituents uniformly spaced along the mannan primary chain. The mannose:galactose ratio has been reported at 1.55 (Morris, 1990) to 1.7 (Grasdalen and Painter, 1980). Locust bean gum (Hui and Neukom, 1964; Sprenger, 1990) differs chemically from guar gum in the higher ratio of mannose to galactose, reported as 3.5 (fewer galactose substituents) (Morris, 1990), and in having a higher DP. Galactose is distributed in copolymer blocks of substituted and unsubstituted mannans. Many more unsubstituted regions exist in the primary structure of locust bean gum than in guar gum, and as a result of the higher mannose content and fewer branch points, this biopolymer is less hydratable than guar gum. However, when heated to 80~ locust bean gum suspensions become transformed to highly viscous dispersions. 0L-Galactosidase removes a higher percentage of galactose from locust bean gum than from guar gum. Only about 20% of guar gum can be hydrolyzed (Hui and Neukom, 1964). Over a solute concentration of 1%, galactomannan pastes have the appearance of a gel. The dispersed solute is mildly pH sensitive between pH 4 and 8; above pH 8, there is a precipitous decline in viscosity. Aqueous dispersions tolerate 10% ethanol. Locust bean and guar gums are good stabilizers of oil-in-water emulsions (Reichman and Garti, 1991), but excessively low concentrations of either may actually destabilize them (Dickinson and Galazka, 1991). A popular use of guar gum is in ice cream manufacture, where allegedly it enhances the smooth texture and slow meltdown property.

II. Chemical Classification

173

Konjac gum contains approximately 85% glucomannan in 1,4-13-D linkage in a glucose:mannose ratio of 1:1.6, and 3 - 4 % acetyl ester groups: there is evidence of branching (Kato and Matsuda, 1973). At 25~ viscosity is about twice the value for guar and locust bean gums at the same concentration (FMC, 1989). The crystallization tendency is encumbered by the acetyl groups: deacetylation (by mild alkali) results in ordering (Tye, 1991) and consequently in a strong, elastic, thermostable gel at concentrations in excess of 1%; higher concentrations yield an irreversible, rubbery gel. Konjac dispersions increase in gel strength after freezing and thawing (Nakao et al., 1991). Konjac flour gum is reported to be pH- and cation-(sodium, potassium, and calcium ions) insensitive; this is consistent with its nonionic character (FMC, 1989), but an isolate did not show a characteristic linear viscosityconcentration profile in water; it did show linearity in the presence of electrolytes (Jacon et al., 1993). The apparent partial specific volume of a 0.2-0.4% dispersion was constant over a wide pH range and increased with increasing temperature from 5 to 50~ it then remained constant (Kohyama and Nishinari, 1993).

C. Fructans Fructose is the repeating monomer in this class of biopolymers, represented by inulin found significantly in dahlia tubers, chicory root, onions, garlic, and Jerusalem artichoke in which the dry matter approximates 60%. All indications are that inulin is a nonreducing, nongelling, hygroscopic fructofuranose containing 1,2-[3- and 2,6-[3-D-glycosidic bonds. In water, inulin undergoes reversion from a more soluble to a less soluble form, in the manner of retrograding starch (Whistler and Smart, 1953): it is slightly soluble in organic solvents. Fructans are easily hydrolyzed by acid. Levan is the branched isomer of inulin.

D. Glycuronans The chemistry of these polysaccharides is dominated by partial esterification of the total number of carboxyl groups. The sequence of uronic acids in the primary structure is occasionally interrupted by rhamnose, and there is usually a trace of acetyl and phenolic substituents. Models developed from experimental data considered neutral side chains with DP = 2-10 (De Vries et al., 1982). These 1,4-a-linked linear uronans are susceptible to alkalimmore so if the C-6 hydroxyl is esterified, but exceptionally acid-stable when this site is unsubstituted. Dispersion stability is less at higher DP. Uronans are endowed with strong dye-fixing and mineral-sequestering properties because of their charged surface.

174

8. Classifications

1. The Pectic Substances

Pectin (unrelated to amylopectin) is the collective name of the galacturonans that are capable of gelling with water, sugar, acid, a n d / o r calcium. High-methoxyl (HM) pectin has DE > 40-50% and low-methoxyl (LM) pectin has DE < 50-40%. HM pectin is the industrial precursor of LM pectin. Demethylation is effected by chemicals or enzymes. Ultrasonication improves the yield of deesterified pectin (Panchev et al., 1994). Completely demethylated pectin is pectic acid. In the wholly protonated form (strongly acidic media), pectic acid quickly precipitates from solution. Aqueous dispersions of HM and LM pectin have very low viscosity (Walter et al., 1985) and tolerate moderate amounts of ethanol (Walter and Sherman, 1983). HM pectin is unique among hydrocolloids in its ability to gel in acidic media (pH < 3) amid a high concentration of sugar (65%). The gelation mechanism is uncertain, but is known to involve stiffening of the primary chains through protonation, water inactivation by high soluble-solids content, and network coupling. LM pectin requires less sugar and gels by cooperative association through calcium bridges. In juice-milk beverages, HM pectin is preferable to LM pectin, because of its insensitivity to Ca 2+. HM pectin jelly is not ordinarily heat-reversible, whereas LM pectin jelly is. HM pectin gels prepared at no higher than 50~ are metastable (Walter and Sherman, 1986). Gelation of sugar beet pectin is a coupling reaction of feruloyl groups with some oxidants in a way that releases free radicals (Thibault et al., 1991).

2. Alginate

Alginate (Cottrell and Kovacs, 1980; Sime, 1990) is a linear, heteropolysaccharide consisting of 1,4-[~-B-mannuronans and 1,4-e~-L-guluronans (alginic acid) in variable amounts, depending on the source. Three blocks of copolymers have been identifiedma mannuronan sequence, a guluronan sequence, and an alternating mannuronan-guluronan sequence. None of naturally occurring algin, alginic acid, or alginate of di- and multivalent cations is dispersible in water, but each swells to a pastelike consistency. Sodium alginate is hot- and cold-water-dispersible, nongelling, and capable of tolerating 30-40% ethanol with proportionately increasing viscosity. Sodium alginate is remarkably freeze-thaw stable, which ensures its application in refrigerated foods. A wide spectrum of properties is possible by manipulating the ratio of Na + to Ca 2+ in mixed salts. The gelling mechanism of alginate is a cooperative association of double helices of guluronate moieties with Ca 2+ (Rees, 1969). Concentration and the manner of addition of Ca 2+ have an influence on gel texture. Alginates with a high guluronan content make strong, brittle, heat-stable gels: mannuronan gels are weak, elastic, and less heat-stable (Kelco, 1986). Calcium

II. Chemical Classification

175

alginate gels are thermally irreversible and dimensionally stable in the manner of a covalent network (Cottrell and Kovacs, 1980). Propyleneglycol alginates do not precipitate from acidic media and their acid stability increases in proportion to the propyleneglycol content. Additionally, those esters with DS > 60% are not precipitated by Ca 2+, because the bulky propyleneglycol substituent hinders the prerequisite alignment for gelation and precipitation. These esters are more tolerant of alcohol than the unesterified alginate. 3. Gum Arabic

Gum arabic (gum acacia) (Whistler, 1993) is a slightly acidic, highly branched, complex glucuronan containing a 1,3-[3-D-galactan main chain and side chains of 1,6-galactopyranose that are themselves substituted with rhamnose, arabinose, and glucuronic acid. This gum is ordinarily watersoluble to approximately 50% concentration, tolerant of ethanol to approximately 60%, incompatible with most other organic solvents, and nongelling. The solution viscosity is very low, even at 40% solute concentration; for this reason, gum arabic dispersions exhibit mostly Newtonian rheology. The dispersions are strongly acid-sensitive over a wide pH range. The dry gum, heated to 170~ and immersed in water, swells without dissolution to a nonsticky gel (Meer, 1980b). The excellent film-forming characteristics of gum arabic make it a favorite encapsulating polysaccharide. Gum arabic is covalently bonded to approximately 2% protein that is believed to confer exceptional emulsifying properties (Dickinson and Euston, 1991a; Randall et al., 1988). Anderson (1988) located varying proportions of amino acids at the periphery and in the interior of the complex. 4. Gum Karaya

This gum (Meer, 1980c; Kubal and Gral~n, 1948) is a processed, partially acetylated galacturonan exudate characterized by strong cohesiveness and adhesiveness, high acid stability, and the usual polyanion sensitivity. Galactose, glucuronic acid, and rhamnose are minor constituents (Dziezak, 1991). 5. Gum Tragacanth

This uronan (Stauffer, 1980; Whistler, 1993) contains a major neutral, insoluble albeit swellable component called bassorin and a minor waterdispersible, acidic component called tragacanthin. It is one of the more acidand heat-stable, surfactant polysaccharides, and it has wide application in a miscellany of industries. The primary structures appear to be arabinogalactans and arabinogalacturonan methyl esters incorporating lesser quantities of xylose, rhamnose, and fucose.

176

8. Classifications

Fractionation of the major and minor components of gum tragacanth is achieved by a variety of techniques, e.g., filtration, centrifugation, fractional precipitation, etc. In this last technique, tragacanthin disperses in 3:1 ethanol:water, while bassorin simultaneously precipitates as a gel. Acid hydrolysis of tragacanthin yields tragacanthic acid (Belitz and Grosch, 1987). 6. Xanthan

Xanthan (Kelco, 1976; Sandford, 1979) is constructed with a repeating five-member unit of three monosaccharides, viz., [3-D-glucose, [3-I>mannose, and glucuronic acid in a 2:2:1 ratio. The main chain is a 1,4-[3-D-glucan, and the side-chains are trisaccharides of or- and [3-D-mannose and glucuronic acid. Acetyl and pyruvate groups are present, but appear to have no role in functionality (Callet et al., 1988). Different concentrations of pyruvic acid in the molecules contribute to different rheologies. Xanthan tolerates 50-60% ethanol, once the ethanol is added after the gum has been dispersed in water; it is hot- and cold-water-dispersible, and disperses directly in glycerol at elevated temperatures (> 65~ NaC1 has little or no effect on aqueous dispersions, except below approximately 0.01% when electroviscosity begins to show. This polysaccharide is exceptionally heat-, acid-, alkali-, enzyme-, shear-, and electrolyte-stable: these stabilities are rooted in the interaction between the main and side chains. Importantly, xanthan develops a highly viscous paste that simulates a weak elastic gel as the dispersed solute gradually loses mobility (cooling) without initiating a phase change. Xanthan dispersions have an identifiable "r0 that is of paramount importance in suspension stabilization. Its exceptional acid stability makes it the stabilizer of choice (over most other glycuronans) for inclusion in fruit juice drinks and vinegar-based dressings. 7. GeUan

Gellan gum (Sanderson, 1990) is a partly acetylated linear heteropolySaccharide whose repeating unit is a tetramer of one 1,3-[3-D-glucose, one 1,4-[3-D-glucose, one 1,3-[3-D-glucuronic acid, and one 1,4-ot-L-rhamnose. Gellan structure and conformation are conducive to crystallite formation (Chandrasekaran et al., 1988b). Functional properties of commercial gellan gum depend on the acetyl content: partial deacetylation is necessary for water dispersibility. This property is augmented both by a low ionic environment and a total conversion of the mixed-cation extract to the monovalent salt. The partially deacylated fibrils behave somewhat as starch granules: they do not disperse in water at low temperatures (below approximately 70~ but do at high temperatures. This polysaccharide is a starch mimetic, given its partial cold-water insolubility and transformation of a suspension to a highly viscous dispersion upon heating (Sanderson, 1990). Viscosity is in-

II. Chemical Classification

177

creased significantly by electrolytes to a state of gelation at room temperature at as low a gellan concentration as hundreths of a percent: a concentration of 0.5% yields the firmest gel. Gellan gels are soft and elastic in low ionic-strength media and firm and brittle in high ionic strength media. The sols show a temperature hysteresis, which develops upon cooling from above 70~ to 20-50~ but no melting until 65-125~ Thermal reversibility and irreversibility are inducible by controlling the cation concentration. Robinson et al. (1991) schematically represented a weak gellan gel held together by helical associations in the absence of cations and a strong gel assembled by cation-mediated aggregates of helices. 8. Hyaluronic Acid

Although not currently a recognized food polysaccharide, hyaluronic acid (Whistler and Smart, 1953) is of interest, inasmuch as it is the strongest of the linearly configured uronans: it is composed of equimolar quantities of N-acetylglucosamine and glucuronic acid. Its importance in living animal tissues is in its lubricity, bonding, and transport properties. Hyaluronic acid complexes with protein and precipitates under acidic conditions.

E. Sulfated Glycans Sulfate in this class confers much greater acidity and dispersion stability than does the carboxyl group in glycuronans. 1. Agar

Agar presents an interesting example of a mixed polysaccharide stabilizer containing variable quantities o f a nongelling uronan and a gelling glucan (agarose; also called agaran). This biopolymer is a linear heterogalactan with 1,3-, 1,4-, and 3,6-ot- and [3-anhydroglycoside linkages. Agaran is neutral and helical with a polar interior (Morris and Norton, 1983). Reportedly it undergoes retrogradation similarly to amylose when isolated (Hayashi and Kanzaki, 1987). Agaropectin is the other major component; it has a low degree of sulfation and small quantities of pyruvic and uronic acids. Agar is cold-water-insoluble, but hot-water-soluble. Its sols are heat-stable with essentially constant viscosity in the neutral pH range, and it is selfgelling at as low as hundreths of a percent; higher sulfation increases the critical gelling concentration (Rees, 1972c). Agar dispersions exhibit temperature hysteresis, whereby a 1.5% heated aqueous sol gels at 32-39~ upon cooling and does not solate until heated again to 60-90~ T m is raised by increasing the concentration; salt hastens the onset of gelation; sugar increases gel firmness and enhances gel transparency (Matsuhashi, 1990);

178

8. Classifications

when cooled to or below 0~ gel properties are lost and the solute precipitates. Agar gels are distinguished from most other hydrocolloids by the sharp, rigid boundary they share with water at an interface, upon cooling a hot sol (Fig. 1 in Chapter 2). Consequently, this biopolymer has been applied on occasion to antisloughing and to sealing c u t fruit and vegetable surfaces. Agar precipitates tannins from wine, and its superb stability to enzymes accounts for its universal use in solidifying microbiological culture media.

2. Carrageenans Carrageenans (Moirano, 1977; Guiseley et al., 1980; Yalpani, 1988; Therkelsen, 1993) are linear heteropolysaccharides structurally related to agar, but have a higher sulfate content. The main chain consists of alternating copolymers of 1,4-ot- and 1,3-[3-D-galactopyranose and 3,6anhydro-D-galactopyranose: pyruvate and methoxyl groups are minor substituents. The natural extract is a mixed salt of calcium, sodium, potassium, and magnesium. Partial desulfation with alkali simultaneously replaces calcium and magnesium with sodium and potassium, and increases hydratability and the gelation tendency. For convenience, the numerous possibilities for sulfation and desulfation within the family of carrageenans are identified by Greek prefixes. The cationic nature and content, the preponderance of glycoside bonding, and the sulfate distributions determine whether the isomer is K-, )t-, or ~-carrageenan; each form has different sol-gel characteristics. In sols, the ordinarily gelling K- and ~-carrageenan aggregate through cation mediation (Rees, 1969; Dalgleish and Morris, 1988). K-Carrageenan gels are firm, brittle, and given to syneresis, but t-carrageenan gels are soft, elastic, and less prone to this defect (Roesen, 1992). Thermoplasticity is based on the reversibility of the double helices, k-Carrageenan is cold-water-dispersible and nongelling in any cationic form. In this structure, the C-2 sulfate performs as a wedge to prevent development of a double helix (Moirano, 1977). Sulfate in K- and ~-carrageenan does not interfere with the orientation, and the double helix is therefore not forbidden; so, on cooling, double helices develop into the building blocks of a three-dimensional network (Rees, 1972a, b, c). Of these three gums, K-carrageenan is most sensitive to electrolytes; all tolerate large quantities of sugar in the order K-, )t-, and ~-carrageenan; acid stability is ~>~>K.

The K - and L-carrageenan gels show a temperature hysteresis between gelation and melting. The sol-gel transition temperature depends more on the specific ion and its concentration than on the carrageenan. Gels may be formed from carrageenan sols without heat (cold process) by permitting Ca 2+ to diffuse into the sol. The higher Tgel of ~-carrageenan has favored its use in the manufacture of food gels that/aeed not be refrigerated.

Iil. Summary

179

Carrageenans complex with protein under conditions where carboxylated polysaccharides do not (Tolstoguzov, 1986). One of their major applications is to stabilize casein in evaporated milk and dairy foods.

3. FurceUaran

Furcellaran is a polysaccharide related to K-carrageenan; it differs mainly in origin and a smaller quantity and narrower distribution of sulfate.

III. Summary With few exceptions, all classes of polysaccharides have c o m m o n heterogeneities that exist across chemical and functional boundaries. In many instances these heterogeneities preclude definitive structure-function' correlations. This notwithstanding, origin, fine structure, etc., can sometimes confer subtle distinctions, so that one polysaccharide may be preferable to others for a specific purpose. Dispersions of wheat starch have lower viscosity than dispersions of corn starch at identical concentrations, whereas waxy maize starch is the bodying agent of choice over corn starch, when concern for clarity is secondary to dispersion stability. Polysaccharides of different origins can have similar structures, and polysaccharides with a c o m m o n origin can have different structures. Classifying them chemically is the least ambiguous system of characterization, but information gleaned therefrom does not inform that guar gum, for example, is cold-water-soluble, but locust bean gum is not, that agar and gellan show similar temperature hysteresis at different concentrations of solute and electrolytes, that low-acyl gellan gum is texturally similar to agar and K-carrageenan and can replace them (Sanderson et al., 1988), and that agar gels behave similarly to starch gels (Oates et al., 1993). Neither does chemical classification suggest a relatively hydrophobic interface between water and ionic agar, alginate and acacia gums, and nonionic konjac. There are hardly recognizable differences between ionic and nonionic polysaccharides where electrolyte sensitivity is not a factor. Moreover, different combinations of stimuli can elicit the same response in different dispersions and, conversely, the same stimulus can elicit a different response in the same class. Regardless of classification, polysaccharides express varying degrees of ethanol tolerance: neutral species are mostly unresponsive to electrolytes, including weak acid, and protonated species precipitate with low solvent retention from strong acid and Ca 2+ solutions. Exceptions to many general rules abound; for example, polyacids are generally the most stable species,

180

8. Classifications

but the C-6 methyl ester uronanmpectinmis unusually more labile than the C-6 alginate ester uronanmpropyleneglycol alginate. Xanthan expands but CMC contracts in dilute electrolyte solutions, after viscoelasticity. Given all the commonalities and criticalities, dispersed polysaccharides can logically be assumed to be functionally interchangeable, irrespective of any systematic grouping.

CHAPTER 9

Saccharides in Fat Replacement I. Introduction Supplementing cellulose, starch, pectin, and the exudate gums, many intermediate DP (100-200) saccharides comprise a sizable fraction of the cell-wall matrix of a terrestrial plant. The concentrations are highest in the seed coats of grain, parenchymatous stems, and the pericarp of fruits and vegetables. These saccharides also display hydrocolloidal activity, are nongelling, and are immune to the action of human digestive enzymes. They are to cereal grain what cellulose is to vegetative tissue. Between the extremes of a simple sugar and a polysaccharide, they span a wide range of and a correspondingly wide distribution of properties: for example, concurrently with their ability to scatter light, a characteristic of polymers, they exhibit strong reducing power. Some properties, e.g., dispersion, transparency, and stability, are superior to those of polymers.

A. Hemicellulose The hemicelluloses are polymolecular, random-coil, often branched heteroglycans interspersed with cellulose microfibrils and starch granules in vivo. Glucose, xylose, and arabinose are the prominent sugars, although lesser quantities of galactan, mannan, etc., and minor concentrations of acetic, ferulic and uronic acids, etc., are in evidence. Heterogeneity extends to dimensions and conformation. The total hemicellulose content of plant materials is 15-30% (Norman, 19431, depending on species, cultural practices, and maturation. Collectively, they are second only to celluose in natural abundance. The noncellulose P-D-glucans in hemicellulose are generally 1,3- and 1,4bonded arabinans, xylans, arabinoxylans, arabinogalactans, etc. Their aqueous dispersions are quite viscous to the detriment of filters used to clarify cereal-based fermented beverages. Oat P-glucans are rheologically

181

182

9. Saccharides in Fat Replacement

similar to guar gum (Autio, 1996). The pentosans and glucans in hemicellulose, by themselves or through association with residual protein, are intensely surfactant: those in cereal flour containing traces of ferulic acid undergo oxidative gelation (Hoseney, 1986).

B. Oligosaccharides Oligosaccharides are intermediate DP hydrolysates of polysaccharides or are synthesized by enzymes. They are empirically differentiated from polysaccharides by a chain length of 2-20 monomers (Munk, 1989); Ikan (1991) limits the differentiation to ten monomers. These species are important moieties in polysaccharide structural elucidation by gas chromatography and mass spectroscopy, because their long segments are identical to those of the original primary chain. By manipulating the reaction variables, the DP can be tailored to functional specifications. Commercial dextrins are specifically the olig0mers of starch. White dextrins, so called because of their visual appearance, are produced from a 30-40% suspension under the mildest possible hydrolysis conditions (79-120~ for 3-8 h in 0.2-2% H z S O 4 o r HC1). Yellow dextrins and British gums are the partial hydrolysates at higher time-temperature integrals. Maltodextrins, dextrose equivalent 2~ 5-19, derive from controlled enzyme or acid partial hydrolysis of gelatinized corn starch. The 20-24 dextrose equivalent hydrolysates are corn syrups (Appi, 1991). Amylodextrin is the homogeneous product of prolonged hydrolysis of starch below Tgz, terminating in the crystalline equivalent of Avicel and approximating 25 glucose monomers. The hydrolysis is normally at room temperature over intervals of months, wherein the amorphous regions are degraded and the starch crystallites are left intact. Unlike Avicel crystallites, starch crystallites may be disrupted by stress (Kerr, 1950). Cyclodextrins (Parrish, 1987; Szejtli, 1981, 1991) are water-soluble, cyclic oligosaccharides (cycloamyloses) formed by the action of glycosyl transferase (from Bacillus macerans) on amylose. The designation, er [3, and y (Schardinger compounds) indicates ring formation with six, seven, and eight glucopyranose sections, respectively, in ascending order of cavity size. [3Cyclodextrin is currently the industrially important oligomer of the three. The existence of "fight helical regions" in amylose has not gone undisputed (Banks and Greenwood, 1975), but the ease of the biocatalytic cyclization

20. Dextrose equivalent equals 100 times the weight of reducing sugar as dextrose per unit weight of dry substance.

II. Isolation

183

reaction gives credence to the concept of the linear segments of starch in helical conformation (Szejtli, 1991). As the product of specific enzyme action, the cyclodextrins are recovered from water in 99% purity. Polydextrose (Appi, 1991) is a synthetic, randomly bonded, amorphous, condensation heterooligomer of glucose, sorbitol, and citric acid, with M not exceeding 5000 Da and a solution pH of 2.5-3.5. Its solution viscosity is slightly higher than that of sucrose at similar concentrations, and it forms a clear melt above 130~ The partial hydrolysis of fructans yields fructooligosaccharides. Aspergillus niger, a fungus, produces fructooligosaccharides in commercial quantities from sucrose. These oligomers, claimed to be 0.4-0.6 times as sweet as sucrose, are indigestible by humans (Spiegel et al., 1994).

II. Isolation Hemicellulose and oligosaccharides are characterized according to their water and alkali solubility: branching and density are the major distinguishing features. Where the lignin content is low, e.g., in the endosperm of cereals, hot water extracts a considerable percentage. In a typical hemicellulose isolation process, finely ground plant material (200 g) is treated with 5-10% sodium hydroxide in oxygen-free water at 25~ for 18-24 h (Whistler and Feather, 1965). The water-insoluble fraction, containing two subfractions differentiated by density, is extractable with higher rather than lower NaOH concentrations: progressively higher concentrations of alkali extract progressively more of the dense saccharides. Considering the heterogeneities of the intermediate DP saccharides (composition, bonding, conformation, DP, etc.), there is no single criterion of purity, but to the extent that extracts can be considered pure, the highest grades are obtained from holocellulose (i.e., cellulose previously treated to remove lipids, starch, pectin, water-soluble carbohydrates, and lignin). Acidification of the alkali supernatant liquid results in a precipitate, hemicellulose A, that is rich in the higher DP water-insoluble xylans and poor in arabinans and uronans, The acidic supernatant liquid retains hemicellulose B, that in contrast to hemicellulose A, has lower DP, higher concentrations of arabinan and glucuronans, and branching (Dekker, 1979; Hoseney, 1986). Hemicellulose B is recovered from the acidic supernatant liquid by precipitation with ethanol. In another fractionation process with water-sodium carbonatebutanol, hemicellulose was separated in the aqueous phase and lignin was separated in butanol, while cellulose remained inert (Magee and Kosaric, 1985).

184

9. Saccharidesin Fat Replacement

III. R e a c t i v i t y

Extraction, purification, and bleaching (with hydrogen peroxide) leave hemicellulose fibers softened, swollen, and with greatly enhanced amorphism and adhesiveness, but with water retentivity (Gould et al., 1990) inferior to polysaccharides, although still quite high. The stability of hemicellulose dispersions is maximum at neutral pH. Dried fibers are difficult to redisperse. The isolated, intermediate DP saccharides are more kinetically mobile than the polysaccharides, because of their smaller size, and they are more chemically reactive, because of the lower ratio of monomer to reducing end groups. Mild acid hydrolysis splits simple sugars from the pentosan primary structure that, on boiling in concentrated HC1, dismutes to 2-furaldehyde (furfural) in three stagesma primary random hydrolysis, followed by monomerization, and finally monomer decomposition. Knirel et al. (1989) used hydrogen fluoride to circumvent certain problems inherent in the dismutation; they claimed that high and changing specificities were a function of reaction temperature (below freezing). The reaction of hemicellulose to heated, concentrated (12%) HC1 is a test of the former's presence. Detection is by the violet or red color developed by one or more of the reaction products (chiefly furfural) with orcinol or phloroglucinol. The coloration is either blue, greenish, or nonexistent with KI.I 2 (Whistler and Smart, 1953). The quantity of furfural produced relates directly to the quantity of pentosan present (Smith and Montgomery, 1959). Under the same reaction conditions, hexoses produce hydroxymethylfurfural. Furfural is steam-distillable; hydroxymethylfurfural is not. Hemicellulose immersed in alkali saponifes, enolizes, deesterifies, depolymerizes by [3-elimination, and oxidizes in the presence of air.

IV. Uses

The food, feed, and paper industries use hemicellulase to convert hemicellulose to useful products (Wong and Saddler, 1993). At the concentrations found in vegetable waste matter, hemicellulose has been viewed intermittently as a potential enzyme substrate and as feedstock for commercial ethanol and furfural production. Pure cellulose makes inferior paper, because cellulose fibers suffer appreciable physical degradation during the pulp beating operation. A certain amount of hemicellulose, especially the uronic acid subfraction, improves quality (Whistler and Smart, 1953).

V. Fat and Fat Replacement

185

The reactivity of hemicellulose and oligosaccharides is an asset in food fabrication. As intentional food additives, these intermediate DP saccharides perform numerous functions, e.g., inhibition of sucrose crystallization, surfactancy, clarification, sweetening, binding, antistaling, anticaking, antibrowning (antioxidation), humidification, and the transport of flavor and aroma. They make thin, opalescent dispersions resistant to settling, because the population of crystallites and the DP are low. The water retentivity of [3-glucans imparts high viscosity and visco-elasticity to their dispersions, thereby enabling them to retain large volumes of gas in doughs and batters--a property that persists throughout baking (Jeltema et al., 1983). The dense, insoluble subfractions swell or can be sheared into similarly viscous and viscoelastic fluids. Hemicellulose has a greater impact on food texture than do polysaccharides (Kim and Kim, 1988), because it is this group of natural compounds that differentiates crispiness in vegetables (Klockeman et al., 1991). The many physical changes it undergoes with time directly affect the texture of cooked plant tissues whose quality attributes remain resident more in the low DP than in the high DP component (Kim and Kim, 1988). One of the arguments for the use of [3-cyclodextrin in food is that it can complex deleterious hydrophobic compounds like cholesterol and caffeine whose concentrations may then be lowered or eliminated (Oakenfull and Sidhu, 1992; Hedges et al., 1993). By the same mechanism, [3-cyclodextrin can emulsify water-triglyceride mixtures (Shimada et al., 1992), protect light-, heat-, and oxygen-sensitive compounds, eliminate undesirable flavors and hazes, and stabilize desirable ones (Miyawaki and Konno, 1982). Hemicellulose and oligosaccharides are now staple items in fat replacement systems. 2a They are claimed to have indirect beneficial roles in human health, because they ferment in the intestines (Tomomatsu, 1994) and large bowel where they produce short-chain fatty acids that seem to enhance electrolyte absorption and stimulate colonic muscular activity (Topping, 1994). Polydextrose is used as a low-calorie bulking agent.

V. Fat and Fat Replacement

A diminution of the quantity of fat in foods, without compensating additions, automatically leads to increased concentrations of the other components: for example, a formula consisting of 80% water, 10% protein, 5% carbohydrate, and 5% fat would automatically have a carbohydrate increase from 5 to 5.3% of the mixture (from 25 to 33.3% of the solid phase), if the fat were 21. Fat substitute, mimetic, and replacer are herein used interchangeably.

186

9. Saccharides in Fat Replacement

completely removed. In this sense, water, carbohydrates, and proteins are fat replacers. A low-fat or fat-free formula cannot have the same gustatory, tactile, textural, and rheological characteristics as a fat-laden formula. Why? Fat contributes uniquely to food through its nonpolar chemical composition, its physical response to low heat (softening), and its interaction with other ingredients. Nevertheless, to fabricate low-fat or fat-free products without appreciable loss of quality, ingredient manufacturers and processors have developed facsimiles of fat, using poly- and oligosaccharides, proteins, and blends. The observations of Campbell (1989) on unsaturated monoglycerides versus saturated glycerides in oil-water stabilization suggest that the less amphiphilic (more ethanol-tolerant) polysaccharides should make better stabilizers and fat replacers. Jones (1996) recommended a "holistic approach" to fat replacement, whereby the spherical shape of a fat droplet as well as the properties of a fat are mimicked by using hydrophobic compounds. Accordingly, the least amphiphilic saccharides with the shortest persistence lengths, e.g., pectin and the cellulose derivatives (Lapasin and Pricl, 1995), are suitable models. By their nature, the intermediate DP saccharides fulfil the requirement.

A. Essential Roles in Food

A constitutively hydrophobic food fat is called upon to perform in a hydrophilic environment, suggesting the most important physical property to be its emulsifying capacity. Simultaneously, the fat is ingredient-compatible, heat-stable, and confers a smooth oral sensation. The basic assignment of a fat replacer is to mimic these properties through substitution for the fat's viscosity, texture, and the slippery, creamy, lubricious mouthfeel (Glicksman, 1991). One of the earliest fat substitutes to perform thus was Simplesse (Roller and Jones, 1996), a protein perceived to be of a creamy texture due to inherent 0.1-3.0-p~m-diameter microparticles (Thayer, 1992). In dough and batter, fat is thought to waterproof starch particles, thereby preventing hardening of the gluten-starch suspension (Platt and Fleming, 1923). This tenderization process is called shortening and the lipid material (usually a special mixture of glycerides) is the shortener. A characteristic of the shortener is that the phase transition from solid to liquid is completed over a narrow plastic range. The brittle texture of cakes and pastries depends on retention or restoration of a certain percentage of fat crystals after baking and cooling. By supplying the stabilizing crystallites and emulsifying a large volume of air and water in dough and batter (creaming), shortenings ensure that the product has a moist yet friable structure. Glicksman (1991) organized fat substitutes into 10 general categories, viz., synthetic fat substitutes, synthetic emulsifiers, hydrocolloids, starch de-

v. Fat and Fat Replacement

187

rivatives, hemicelluloses, [3-glucans, soluble bulking agents, microparticulates, composite materials, and functional blends. Edible chemical compounds consisting of carbon backbones with carboxyl and methylcarboxyl groups as ester and acyl substituents have been disclosed as fat mimetics-some with special advantages (Klemann and Finley, 1989). Saccharides are identifiable with this composition are not ordinarily metabolized by humans, and hence have no calorie content. Gums, maltodextrin, corn syrup, etc., impart "body, cling and firmness" (Yackel and Cox, 1992) in fat replacement. A fat contains 9.54 kcal g - l , a simple sugar contains approximately one-half that amount, and a polysaccharide is not metabolized; therefore, the calorie contribution of an intermediate DP saccharide fat substitute is proportional to the degree of saccharification. B. Carbohydrate Fat Mimetics Carbohydrate fat mimetics (Thayer, 1992) should be thermoreversible gels (e.g., starch) and microreticulated fibers (e.g., cellulose). As gels, they should provide plasticizing moisture, and as fibers, they should create structural rigidity. Concentrations higher than normal use levels are necessary for them to mimic the fatty sensation. They may be dry-blended with the rest of the formula, or suspended in water and sheared separately. A patented product of oat fiber, called Oatrim, containing up to 10% [3-glucans, is considered to have fat mimetic as well as cholesterol-lowering capabilities (Roller and Jones, 1996). Pectin (Mangat, 1992), starch, alginate (Kerner and Ward, 1992), maltodextrin (Inglett and Grisamore, 1991), dried fruits (Silge, 1992), cereal, vegetable fibers (Glicksman, 1991), and enzymehydrolyzed rice (Sander, 1992) all have been reported to be fat replacers. One of the suggested advantages of hydrolyzed rice solids over most other carbohydrates is that their cold-water gel does indeed resemble a fat. The protein adjunct in rice and other mimetics of natural origin augments the fatty sensation. Other reported fat substitutes are xanthan contributing appropriate viscosity (Duxbury, 1993), high-temperature curdlan gels reputed to have a high fat-adsorbing capacity (Harada et al., 1993), and polydextrose (LaBell, 1991) functioning also as a sugar substitute. Methylcellulose, hydroxypropylcellulose (Dziezak, 1991; Henderson, 1989), and microcrystalline cellulose (Penichter and McGinley, 1991), while imitating fat in stabilizing creams and toppings, are at the same time barriers to lipid migration in fried foods, while promoting adequate moisture retention in the product. A single polysaccharide can rarely optimize all the requisites of a fat, so the trend has been toward combinations, e.g., konjac-carrageenan blends in lean meat and sausages (FMC, 1991). Simulation should be more achievable by the use of cosolutes with complementary sol-gel characteristics. The

188

9. Saccharides in Fat Replacement

substituted celluloses are hydrated and fully viscous at room temperature, but in the vicinity of 50~ they phase-separate abruptly. By combining any of the substituted celluloses with a normal gelling polysaccharide, e.g., gellan acting like starch, upon heating, the sol-gel transition of the former would be complemented by the latter's gel-sol transition, which would result in deposition of crystallike, stabilizing substituted cellulose particles embedded in a continuous gellan phase. Protein-polysaccharide (i.e., gelatingalactomannan) antagonism has created similar fat mimetics, consisting of gelatin particles embedded in a continuous galactomannan phase (Muyldermans, 1993), possessing plastic textures different from those of either antagonist. A guar gum-microcrystalline cellulose blend is a superior fat replacer (FMC, 1993). Micronization is an optional process that helps to reproduce the fatty sensation afforded by polysaccharides. The 0.2-nm particles in Avicel (FMC, 1993) and the 1-5-nm particles in the rice mimetic (Pszczola, 1991) simulate lipid emulsion rheology as well as lipid oral sensations. The simulation mechanism implicates a weak gel structure and an expansive surface where a large volume of water is immobilized.

VI. Summary The diverse properties of saccharides have made them the subjects of research and development in the contemporary quest for natural fat substitutes. It is now apparent that there will be no single fat mimetic for all applications; so synergistic and antagonistic interactions between saccharides and with protein show promise for specific applications. The exceptional performance of intermediate DP saccharides, currently of limited industrial value, can fulfil a critical role as they have in the past in other industries.

Appendices Appendix I Unit of Viscosity V i =kr4t'r/(~qi'l ) cm3 = k ( c m 4 s g cm)(cm2

S2

' l ] i - k ( c m 4 s g c m ) ( c m 3 cm 2 "q~-

kg cm

-1

s

cm 'rl/ cm3) -1 S2

cm)

-1

-1

lfi= cm 3 r = cm t=s

I" = dyn A -1 = [g c m ( c m 2 s 2) -1]

Appendix 2 The Schulz-Blaschke Equation

~.~lc =

[~] + k"[~]~s~

= [,~](1 + k"~s.)

= qsp/(1 + k'-qs~)C

[~l ~ = {(1 + k,,,nspl/~sp}C

189

Appendices

190

Appendix 3 The Maxwell Model ( H o o k e ' s law)

Eto t = 'I'//G

(~tot// = T/T])

(Newton's

law)

Etot ---- ( ' r / q q ) / ~ t o t - - T / G "4- (T/~)t

O~.tot/Ot "-- ( 1 / G ) (O'rlOt) + "r/T] ( l / G ) O'rx/Ot = ( O E t o t / O t ) -- ( ' r / ' q ) O"r/Ot = G[ ( OEtot/Ot ) -- ( T / T I ) ] = shear t = time -r = stress G -m o d u l u s o f elasticity T] = viscosity

Appendix 4 Unit of ~1/ G From Newton's equation, T] = dyn s c m - 2 -- g c m s - 2 9 c m - 2 - - g c m -1 s -1 .

(A4.1)

From Hooke's equation, G = dyn c m - 2

-- g c m ( c m 2 s 2) =gcm

-1

-1 s - 2 .

By r a t i o n a l i z i n g A5.1 a n d A5.2 T]/G ~ g cm ~s,

- 1 s - l ( g c m -1 s - Z ) -1

(A4.2)

Appendix 6

191

Appendix 5 The

Voigt- Kelvin Model

r = Ge

(Hooke's

equation)

E = "r/G

(Newton's

r = "qe/t

equation)

e -- "rt/~l "~ = ~ l e / t + Ge

r - Ge

Tle/t

=

e / t = ( -r - G e ) / ~ 1 e / t = "r/~q -- Ge/+q

- Ge/qrl

Oe/Ot='r/~l

e = ('r/" q

- -

Ge/'q)t

e - shear t = time r = stress G = modulus

of elasticity

-- v i s c o s i t y

Appendix 6 The Mark-Houwink Equation and the Hydrodynamic Volume [,],Mi

=

l o g [ ~111 + l o g M 1 = l o g [ TI]2 + l o g M 2 [+q] = K M

v

l o g [ TI] = l o g K + v l o g M

Appendices

192 S u b stitu tin g [~1] f r o m Eq. (A6.4) for [~111 a n d ['I]]2 in Eq. (A6.2) log K 1 + v I log M 1 + log M 1 -- log K 2 + 1)2 log M 2 + log M 2 log K 1 + ( v I + a ) l o g m I - ( v 2 + 1)log m 2 + log K 2 log M 2 = {(v I + a ) l o g M 1 + log K 1 - l o g

K z } / ( v 2 + 1)

log M z ( v 2 + 1) = ( v I + a ) l o g M 1 + log(K1/K2)

log m2 - {(Vl + 1)log ml + log(K~/K2)}/(v2 + 1) = {(v I + 1)log M 1 } / ( v 2 + 1) + l o g ( K 1 / K z ) / ( v

2 + 1)

= {(v, + 1)log M 1 } / ( v 2 + 1) + ( l / ( v 2 + 1))(log(K1/K2) ). W h e n v I = 112 a n d K 1 - - K 2 , log M 2 -- log M 1 .

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Index

Acetylated starch, 167 Acid, polysaccharides and, 21 Acid-detergent fiber (ADF), 148 Activation energy, of viscous flow, 84 Additivity, polysaccharide-polysaccharide interaction, 102-103 Adsorbates, polysaccharides as, 38-40 Adsorbents, polysaccharides as, 38, 40 Adsorption potential, 126 Aerosols, 63 Agar chemical classification, 177-178 conformation, 9 properties, 161 - 163, 179 uses in foods, 163, 164, 178 Agarose boundary with water, 29, 30 conformation, 10 Aggregation, 25, 48 Aging pilot plant quality control, 149 polysaccharide dispersions, 66-67 Alginate chemical 9 classification, 174-175 gelation, 10, 174 properties and uses in foods, 161-163 uses in foods, 164 Alginate-pectin blend, synergism, 105 Alginic acid conformation, 9 enzyme action on, 23 Alkali, polysaccharides and, 21-22 Alkali earth metals, complex with polysaccharides, 107 Aluminum, complex with polysaccharides, 107 Amadori rearrangement, 120 Amorphous cellulose, phenomenology of, 20 Amorphous polymers, sol-gel transition, 55 Amorphous polysaccharides, 84

Amorphous starch, phenomenology of, 20 Amphipathicity, 4 Amphiphiles, 23, 36 Amphiphilicity, 4 Amylases a-amylase, 23, 106 B-amylase, 23 carbohydrase technology, 24 Amylodextrin, 182 Amylomaize, 165 Amylopectin alkalization, 22 antagonism, 113 branching,. 37 dispersion, 24 phenomenology, 20 in vegetables, 165-167 Amylose alkalization, 22 antagonism, 113 branching, 37 conformation, 10 crystallite formation, 166 dispersion, 24 fatty acid complexation, 108 nominal width, 12 in vegetables, 165-167 Amylose-lipid complex, 106 Aniline blue, 139 Anion exchange chromatography, 127 Anisotropy, polysaccharides, 17 Antagonism, polysaccharide dispersions, 113 Anthrone, 139 Antibiosis, 115 Antifoaming agents, 66 Antithixotropic fluids, 56 Arabic acid, conformation, 10 Arrhenius equation, 84 Association hypothesis, 6 Atactic distribution, 4

223

224 Attraction, 9 Coulumbic, 8 mutual, 6 van der Waals, 29 " Autocorrelation, 89 Avicel-RC, 170

Barium, complex with polysaccharides, 107 [3-elimination, polysaccharides, 21-22, 23 Bingham body, 57 Blending, 102-104, 155 Blending chart, 155 Block copolymerization, properties and, 18 Boltzmann factor, 54 Boltzmann law, 50 Bonding, polysaccharide-water interactions and, 36-37 Born exponent, 43 Boron, complex with polysaccharides, 107 Bostwick consistometry, 77, 150 Branching polysaccharides, 90 polysaccharide-water interactions, 37 Bridging flocculation, 65 British gums, 119, 182 Broken-curve heating, polysaccharides, 12, 167 Brownian motion, 42 Brunauer-Emmett-Teller equation, 39 Bulk modulus, 80 Buoyancy factor, 97

Calcium, complex with polysaccharides, 107 Capacitance, 46, 66 Capacitor, 46 Capillary electrophoresis, 126 Caramels, 120-121 Carbohydrase technology, 24 Carbohydrate fat mimetics, 187-188 Carboxymethylcellulose (CMC) conformation, 9 dispersion, 26 food-grade CMC, 171 heating and, 117 properties, 24, 180 uses in foods, 164 Carrageenan-konjac blends, 187 Carrageenan-phospholipids, 106

Index Carrageenans, 3 alkaline pretreatment, 22 chemical classification, 178-179 conformation, 10 gelation, 10, 11, 37 gels, 27 u-carrageenan, 10, 158-160, 178 K-carrageenan, 10, 11, 22, 104, 158-160, 178 k-carrageenan, 158-160, 178 properties, 158-160 radiolysis, 25 sugar and, 26 synergism, 104 uses in foods, 158-160, 164 Casein-alginate combination, synergism, 110 Cellobiose, solubility, 36-37 Cellulon, 169-170 Cellulosate, 22 Cellulose, 1 alkali and, 22 oL-cellulose, 124 amorphous, 20 chemical classification, 168-171 colorimetry, 138 conformation, 9 crystalline forms, 19 extraction, 124-125 nominal width, 12 phenomenology, 20 regenerated cellulose (RC), 170 xerogels, 63 Cellulose derivatives, 171 Cellulose gum, properties and uses in foods, 161-163 Charge, polysaccharides, 126 Charles-Boyle law, 41 Chemisorption, 39 Chitin conformation, 9 properties, 171 reactivity, 25 Chitosan conformation, 9 reactivity, 25 Chlorozinc-iodine, 138 Circular dichroism spectroscopy (CD), 128, 140-141 Clausius-Clapeyron equation, 49 Cloud point, 16, 165 Cloudiness, polysaccharides, 16, 55-56 Clusters, 12, 73 Cluster theory, 61 Coacervation, polysaccharides, 67

225

Index

Coagulation, polysaccharides, 65 Coalescence, polysaccharides, 65 Coefficient frictional, 77 Huggins interaction, 77, 88 partition, 134 second virial, 85, 112 Coefficient of volume expansion, 34 Coil free-draining, 92 non-free-draining random, 165 random, 8 Coil overlap parameter, 74 Coil-stretch deformations, 117-118 Colloid, defined, 12 Colloidal activity, polysaccharides, 12-17 Colloidal stability, DLVO theory, 42 Colorimetry, 138-140 Complementarity, polysaccharide-polysaccharide interaction, 104 Complex coacervation, 67 Complex modulus, 83 Complex viscosity, 83 Concentrated regime, 73 Concentration and conformation, 11 of polysaccharides in foods, 164-165 and scattering, 18 Concentration regimes, 71-72 concentrated regime, 73 dilute regime, 72-73 mathematical modeling, 74 activation energy of viscous flow, 85 Hooke's equation, 80-84, 190 Huggins equation, 77-78 Kraemer equation, 78 Martin equation, 78 Newton equation, 79, 80, 190 Poiseuille equation, 75-77 power-law equation, 79 Schulz-Blaschke equation, 78-79, 189 Stokes equation, 75 semidilute regime, 73-74 Configuration, polysaccharides, 6-12 Configurati0nal entropy, 50 Conformation, 6-10, 88, 128 analytical techniques, 128 concentration and, 11 polysaccharides, 6-12, 28, 53 random coil, 10, 69, 88, 101-102 solvents and, 8-9 Congo red assay, 138-139 Conjugate solution, 111 Conjugation, 127

Consistometry, 150 Consolute temperature, 55 Constant dielectric, 46 ionization, 44 Constitutive properties, 4 Contact electrification, 63 Contact points, 73 Contour length, 90-91 Contraction, water, 34 Cooperative association, 105, 174 Coordinate orientation, 133 Coordination number, 31 Copper, complex with polysaccharides, 107 Corn starch complementarity, 104 water in, 35 Cotton, extraction and purification, 124 Coulombic attraction, 42 Coulomb's law, 42 Counterions, 22 Coupled network, 102 Covalent gels, 60, 62 Crankshaft motion, 54 Creaming, 66 Creep, 80, 82 Creep test, 80, 81 Critical micelle concentration, 71, 72 Crosslinking, 25, 160 Crude fiber (CF), 147 Cryoprotection, 110 Cryostabilization, 110 Crystallite(s), 12, 18, 20, 25 melting, 12, 55 Cumulant analysis, 89 Curdlan complementarity, 104 conformation, 9, 10 crystalline forms, 19 gelation, 37, 50 helix-random-coil transition, 26 nominal width, 12 properties, 172 syneresis, 67 Curve, stress relaxation, 81 Cyclodextrin-lipid complexes, 108 Cyclodextrins, 108, 182-183, 185

D Dalton, 5 Deacetylation, 22 Debye length, 46

226

Debye scattering, 17, 86 Deesterification, 24 Defibrillation, of a polysaccharide, 57 Degree of carboxylation, 45 of esterification, 4, 20 of polymerization, polysaccharides, 5-6 of substitution, 20 Depletion flocculation, 65 stabilization, 65 Derjaguin-Verwey-Landau-Overbeek (DLVO) theory, 42 Desorption, 38 Destabilization defined, 64 electrokinetic mechanism, 43 flocculation, 65 shelf life, 67 Dextran, 3 chemical classification, 168 crystalline forms, 19 Dextrin, 182 British gums, 119, 182 conformation, 11 gelation, 72 limit, 23 white dextrin, 119, 182 yellow dextrin, 119, 182 Dextrose equivalent, defined, 24 Diastase, 23 Dielectric, 46 Dielectric constant, water, 31 Dietary fiber, 147 Differential scanning calorimetry (DSC), 141, 142 Differential thermal analysis (DTA), 141, 142 Diffusion, 47, 51 polysaccharides, 51-52 Diffusion coefficient (diffusivity), 3, 51 Dihydroxynaphthalein, 139 Dilute regime, 72-73 Dimensional heterogeneity, 18 Disorder-order transition, polysaccharides, 12 Dispersibility micromolecules and, 25-27 polysaccharides, 12-13 Dispersions, 13 additivity, 101-112 antagonism, 113 complementarity, 101-112, 104 dilute, 73 electrostatics and electrokinetics, 42-47

Index

interactions, 101-102, 113 amylose clathrates, 108 cyclodextrins, 108 polysaccharide-lipid, 105-107 polysaccharide-metal, 107-108 polysaccharide-polysaccharide, 102-105 polysaccharide-protein, 108-112, 113 phase separation, 69 polysaccharide-water interactions, 35-36 powdered polysaccharide, 38 sedimentation, 68 stability, 113, 118 total energy content, 43 Dissociation, ionic polysaccharides, 44-45 Dissociation constant, 44 Dissymmetry, 87 Dissymmetry coefficient, defined, 17 DLVO theory, 42 Donnan distribution, polysaccharides, 47, 135 Doppler effect, 89 Dynamic light scattering, 89 Dyne, 76

Elasticity, 15, 16, 74, 80, 83, 190 Elastic modulus, 80 Electric double layer, 45-46 Electrodialysis, 47 Electrokinetics, polysaccharides, 42-47 Electrolyte criticality, 55 Electrolytes, 55 polysaccharide dispersion and, 25-26 Electroosmosis, polysaccharides, 47 Electrophoresis, 46-47, 126, 129 Electrophoretic mobility, 47, 126 Electrostatics, polysaccharides, 42-47 Electrostatic stabilization, 65 Electroviscosity, 57, 59, 127 Elongational viscosity, 16 Emulsan, 106-107 Emulsifiers, defined, 17 Emulsions, polysaccharides and, 62, 66 Encapsulation, 68, 108 Enthalpy, polysaccharides, 48-49 Entrapping technology, 69 Entropy, defined, 43, 49 Enzymes, polysaccharides and, 23-24, 140 Equilibrium metastable, 64, 111 stable, 111 thermodynamic, 64 unstable, 111

Index

227

Equivalent hydrodynamic sphere, polysaccharides, 53-54, 74, 89 Excluded volume effect, 31 Expansion, water, 45 Expansion factor, 27 Extensional viscosity, 16 Extensive properties, 4 Extraction, polysaccharides, 123-125

F Fat-free foods, 186 Fat replacements, 185-188 Oatrim, 187 rice fat mimetic, 187, 188 Simplesse, 186 Fatty acid complexation, amylose, 108 Fermentation, enzyme action in, 23 Fiber, 147 Fiber analyses, 148 Fick's first law of diffusion, 51 Field-flow fractionation, 130 Films, polysaccharides, 63, 73 Flavor-releasing polysaccharides, 2-3 Floc, 62, 63, 73 Flocculating power, 46 Flocculation, 65 Flocculation value, 56 Flory-Huggins interaction parameter, 112 Flow activation energy of viscous flow, 84 birefringence, 35 complex, 73 laminar, 1, 5 Newtonian, 56, 72 plastic, 57 streamline, 15, 16 time-dependent, 75 turbulent, 16, 77 Fluid(s) dilatant, 56 rheopectic, 56 shear-thickening, 56 shear-thinning, 56 thixotropic, 56 viscoelastic, 58 Flux, 51 Foam, 62 Foams, polysaccharides and, 62, 66 Foam stabilizers, defined, 17 Food-grade CMC, 171

Food processing quality control, 148-150, 155 thermal processing, 115-121 Foods. See also Fruits; Vegetables dispersion additivity and complementarity, 101-112 encapsulation, 68-69 fat-free, 186 fat replacement, 185-188 gels, 60 instant food items, 167 low-fat, 186 polysaccharide-protein blend, synergism, 110 polysaccharides food suspensions, 64 properties and uses, 158-163 uses in, 158-165 stability kinetic stability, 64 moist foods, 33 thermodynamic stability, 64 syneresis, 67 xerogels, 63 Force(s), 6 attractive, 31 intermolecular, 6 long-range, 31 repulsive, 8 short-range, 31 Fractal aggregates, 60 Fractal dimensionality, 94-95 Free-draining coil, 92 Free energy of mixing, polysaccharides, 49-51 Free volume, polysaccharides, 54 Freely-jointed chain, 6, 90 Frequency factor, 84 Freundlich equation, 147 Frictional coefficient, 45, 52, 75 Fructans chemical classification, 173 partial hydrolysis, 183 Fructooligosaccharides, 183 Fruits, 3 mealiness, 3 vacuum infusion of sugar, 26-27 Functional groups, polysaccharides, 127-128 Furcellaran, 74, 179 Furfural, 184

228

Index

G Galactomannan-gelatin blend, 188 Galactomannans colorimetry, 139 detection, 140 heterogeneity, 18 pastes, 172 stability in alkali, 22 synergism, 104 Gamma-radiation, polysaccharides and, 25 Gas chromatography (GC), 144 Gegenions, 22 Gelatin, uses in foods, 160-161,165 Gelatin-agar blend, synergism, 110 Gelatin-amylopectin blend, antagonism, 113 Gelatin-galactomannan blend, 188 Gelatin-gellan combination, 110 Gelatinization, 3, 11, 52, 55, 166 defined, 11 reversible, 166 starch, 52, 166 sugar and, 26 Gelation, 60, 101 alginate, 10, 174 branching and, 37 carrageenans, 10, 11, 37 cluster theory, 61 curdlan, 37, 50 egg box model, 7, 55 gellan gum, 72 guar gum, 37 konjac gum, 60 locust bean gum, 37 oxidative, 62 pectic acid, 72 pectin, 11, 24, 52, 72, 174 polysaccharides, 10-11 reversible, 73, 83 scleroglucan, 37 thermal, 55 xanthan-locust bean gum, 105 Gelation temperature, 55 Gel chromatography, 129 Gellan gum chemical classification, 176-177 complementarity, 104 gelation, 72 properties, 179 salt, effect of, 26 Gels, 2, 11,59, 60 covalent, 62 classes, 60-61 ionotropic, 6

junction-zone heterogeneity, 18 physical, 60 polysaccharides, 2 rupture sites, 102 sol-gel transition, 55 syneresis, 19, 67 thermoplastic, 12 Gibbs adsorption, 38 Gibbs free energy, 50 Glucans 0L-D-glucans, 165-168 [3-D-glucans, 168-173 conformation, 10 Oatrim fat mimetic, 187 Glucoamylase, 24 Glucose dialdehyde, 23 Glucose isomerase, 24 Glutamic acid, conformation, 8 Glycogen, 1 alkalization, 22 branching, 37 Glycomannans, properties, 36, 172-173 Glycosides, 21, 37 9 Glycosylation, 120 Glycuronans chemical classification, 173-177 stability in acid, 21 Guar gum chemical classification, 172 compatibility with other polysaccharides, 37 complementarity, 104 conformation, 9 deacetylation, 22 detection, 140 gelation, 37 heating and, 117 properties, 160-161, 179 pseudoplasticity, 74 uses in foods, 160-161, 164 Gum arabic chemical classification, 175 conformation, 9 emulsion stability, 109 properties, 1, 160-161 uses in foods, 160-161, 165 Gum ghatti, conformation, 9 Gum karaya, chemical classification, 175 Gum tragacanth chemical classification, 175-176 enzyme action on, 23

229

Index

H Hairy regions, 18 Heat capacity, 49 Helix, 11, 17, 50 Helmholz double layer, 45 Hemicellulase, 184 Hemicellulose, 124, 181-182 fat replacement with, 185-188 isolation, 183 properties, 184 reactivity, 184, 185 Heterogeneity, 18, 37 defined, 22 polysaccharides, 18 polysaccharide-water interactions, 37 High-methoxyl-pectin, 61, 158-160, 174 High-pressure liquid chromatography (HPLC), 129 High-temperature pyrolysis, 119-120 Hofmeister series, 56 Holocellulose, 124 Homogeneity, polysaccharides, 18 Homogenization, polysaccharides, 27 Homopolymers, homogeneity, 18 Hooke's equation, 80-84, 190 Hooke's law, 80, 190 Host-guest reaction, 108 Huggins equation, 77-78 Humectants, 15 Hyaluronic acid, 44, 177 Hydration, 166 artificial branching and, 37 polysaccharides, 29, 69 water of hydration, 14 xanthan, 37 xerogels, 19, 60 Hydrocolloids, 13 surface tension, 15 Hydrodynamic radius, 89 Hydrodynamic interaction, 35 Hydrodynamics, polysaccharides, 53-54 Hydrodynamic volume, 92-94, 191 Hydrogel, 61 Hydrophile-lipophile balance, 62 Hydrophilicity, polysaccharides, 13-15, 146-147 Hydrosol, 61 Hydroxyalkylcellulose, 24 Hydroxyethylstarch, 167 Hydroxypropylcellulose as fat mimetic, 187 phase-separation temperature, 26

properties, 171 sugar and, 26 Hydroxypropylstarch, 167 Hysteresis, polysaccharides, 19, 59, 82 Hysteresis loop, methylcellulose, 19, 20

I Imaginary shear plane, polysaccharides, 53 Inclusion complex, 130, 141 Induced kinetic stability, 64 Industrial caramel, 120-121 Infinite network, 61 Infrared spectroscopy, 127-128 Inherent stability, 53 Instant food items, 167 Intensive properties, 4 Interaction coefficient, Huggins, 77 lock-and-key, 105 parameter, 50, 112 potential, 42 Interference, 88 Inulin, solubility, 37 Ionic polysaccharides, 43-47, 157 Ionic strength, 9, 46 Ionization, 31, 43, 44 constant, 44 polysaccharides, 44-45 water, 31-32 Ionizing groups, polysaccharide-water interactions, 37 Ionotropic gels, 61-62 Iron, complex with polysaccharides, 107-108 Irreversible thermodynamics, 51 Isotactic distribution, 4

1 Joule, 50 Junction-zone heterogeneity, 18 Junction zones, 18, 52, 60, 62

K Kelvin-Voigt test, 82 Kinetics, 51-52 first-order, 52 pseudo-, 52 Kinking, 6, 7 Konjac-carrageenan blends, 187 Konjac flour gum, chemical classification, 173

230

Index

Konjac gum blends, 104 chemical classification, 173 gelation, 60 Konjac gum-starch blends, synergism, 104 Konjac mannan gums conformation, 9 interaction with xanthan, 12 Kraemer equation, 78 Kulolo, 33

Laminar flow, 15-16 Langmuir equation, 99 Laser diffractometry, 130 Law of cosines, 86, 90 Law of distribution of molecular velocities, 54 Light scattering, polysaccharides, 16-17, 86-90, 136-137 Light-scattering photometry, 136-137 Linear flow, polysaccharides, 74 Linear polysaccharides, hysteresis, 19 Lipase, 106 Lipid conjugates, 106-107 Lipid-cyclodextrin complexes, 108 Lipid-polysaccharide interactions, 105-107 Lipids, nature and properties, 106 Liquid sweeteners, 24 Locust bean gum compatibility with other polysaccharides, 37 detection, 140 gelation, 37 properties, 161-163, 179 synergism, 104 uses in foods, 161-163, 164 Low-fat foods, 186 Low-methoxyl-pectin, 146, 158-160, 174 Low-pressure liquid chromatography (LPLC), 129 Low-temperature pyrolysis, 118-119 Lyotropic series, 56

M Macromolecule, definition, 12 Magnesium, complex with polysaccharides, 107 Maillard degradation, 120, 139 Malt, 23 Manapua, 33 Manna, crystalline forms, 19

Mark-Houwink equation, 91-92, 191 Martin equation, 78 Mass spectrometry (MS), 145 Mass-volume-pressure-temperature relationships, polysaccharides, 41-42 Mathematical modeling concentration regimes, 74 activation energy of viscous flow, 85 Hooke's equation, 80-84, 190 Huggins equation, 77-78 Kraemer equation, 78 Martin equation, 78 Newton equation, 79, 80, 190 Poiseuille equation, 75-77 power-law equation, 79 Schulz-Blaschke equation, 78-79, 189 Stokes equation, 75 molecular size, 85, 100 contour and persistence length, 90-91 fractal dimensionality, 94-95 hydrodynamic volume, 92-94, 191 light scattering, 86-90 Mark-Houwink equation, 91-92; 191 sedimentation, 59-98 surface area, 98-99 van't Hoff equation, 58 Maxwell model, 74, 80, 81, .190 Mealiness, in fruit and vegetable, 3 Mean free path, 42 Melting, 12, 44 Membrane osmometry, 85, 135-136 Metal-polysaccharide interactions, 107-108 Methylcellulose boundary with water, 29, 30 chemical classification, 171 complementarity, 104 as fat mimetic, 187 hysteresis loop, 19, 20 properties, 171 surfactancy, 17, 36 Methylcellulose-starch blends, 104 Microcrystalline cellulose, 187 Microencapsulation, 68 Microheterogeneity, 18 Micromolecules, dispersibility and, 25 Micronization, 188 Modified starch, 167-168 Modulus of elasticity, 80, 83 Molecular encapsulation, 108 Molecular size, polysaccharides, 85, 100 analytical techniques, 130-137 mathematical modeling, 85-100 Molecular structure, analysis, 144-146

231

Index

Molecular weight, polysaccharides, 4-5, 130-137 Monodisperse polymer, 18

N Native cellulose, 169 Native starch, 165 Negative adsorption, 38 Nephelometry, 86 Neutral-detergent fiber (NDF) value, 148 Neutral dextrin-neutral amylose, antagonism, 113 Neutral interaction, defined, 35 Neutral locust bean gum-low-methoxyl pectin, 113 Newton, 7 Newton equation, 79, 80, 190 Newtonian flow, 56, 79 NMR spectroscopy, 140-141 Nominal width, defined, 12 Nonionic polysaccharides, 42-43, 157 Nonsolvent, 9

O Oligosaccharides, 182-183, 185-188 Optical activity, polysaccharides, 17 Order-disorder transition, polysaccharides, 12 Order of reaction, polysaccharides, 52 Orientations, 7 Oscillatory shear rheometry, 83 Osmotic migration, 38 Osmotic pressure, 5 Ostwald ripening, 67 Oxidants, polysaccharides and, 22-23 Oxidative gelation, 62 Oxidized celluloses, 23 Oxidized starch, 23

Paper, 169 Parallel ordering, 6 Parallelism, 7 Pascal, 76 Pastes, 62, 172 Pectic acid gelation, 72 stability in acid, 21

Pectin boundary with water, 29, 30 chemical classification, 174 colorimetry, 139 conformation, 10 dissociation, 45 enzyme action on, 23 extraction, 125 gelation, 11, 24, 72, 174 heterogeneity, 18 high-methoxyl-pectin, 61,158-160, 174 homogeneity, 18 isolation, 25 jelly-making, 26, 27 low-methoxyl-pectin, 146, 158-160, 174 oxidative gelation, 52 stability in acid, 21 syneresis, 67 uses in foods, 164 Pectin-alginate blend, synergism, 105 Pectin-aluminum complex, 107 Pectin jellies, 26, 27, 67 Pentosans oxidative gelation, 62 structure, 145 Peroxidase, 140 Persistence length, 90, 91 Phase changes, polysaccharides, 55 Phase separation, 50 polysaccharide-protein blend, 110-112 polysaccharides, 66-67, 69 Phosphated starch, 167 Photon correlators, 130 Physical gels, 60-61 Pilot plants, quality control, 148-150, 155 Plastic fluid, 57 Plateau value, 38 Poi, 33 Poise, 75 Poiseuille equation, 75-77 Polydextrose, 183, 185 Polydisperse polymer, 18, 89 Polydispersity, polysaccharides, 129-130 Polyelectrolytes, salting in/salting out, 26, 46 Polymers, light scattering and, 86-90 Polymorphism, polysaccharides, 18-19 Polysaccharide analysis, 125-130, 155 aging techniques, 149 anion exchange chromatography, 127 Bostwick consistometry, 150 charge detection, 126-127 circular dichroism spectroscopy (CD), 128, 140-141 colorimetry, 138-140

232

Polysaccharide analysis (c0ntinued) conformation, 128 conjugation, 127 consistometry, 150 differential scanning calorimetry (DSC), 141, 142 differential thermal analysis (DTA), 141, 142 electrophoresis, 46-47, 126, 129 electroviscosity, 127 fiber assays, 147-148 field-flow fractionation, 130 functional group identity, 127-128 gas chromatography (GC), 144 high-pressure liquid chromatography (HPLC), 129 hydrophilicity, 146-147 infrared spectroscopy, 127-128 laser diffractometry, 130 light-scattering photometry, 136-137 mass spectrometry (MS), 145 membrane osmometry, 135-136 molecular weights and sizes, 130-137 NMR spectroscopy, 140-141 photon correlators, 130 pilot plant quality control, 148-150, 155 polydispersity, 129-130 pyrolysis, 145 reducing end-group analysis, 131-132 rheometry, 83, 132-134, 155 sedimentation equilibrium, 95-97, 137 sedimentation velocity, 97-98, 137 sediment volume techniques, 149-150 size exclusion chromatography, 134-135 spectrophotometry, 138-140 structure, 144-146 supercritical fluid chromatography (SFC), 145 syneresis, 150 texture, 150 thermal analysis, 141-144 thermal gravimetry (TG), 141, 142 thin-layer chromatography (TLC), 129 viscometry, 132-134 volume fraction, 146 zeta potential, 126-127 Polysaccharide esters, partial demethylation, 22 Polysaccharide-lipid interactions, 105-107 Polysaccharide-metal interactions, 107-108 Polysaccharide-polysaccharide interaction additivity, 102-103 complementarity, 104 synergism, 104-105

Index

Polysaccharide-protein blends, 108, 109-112 Polysaccharides acid and, 21 as adsorbates, 38-40 as adsorbents, 38, 40 aerosols, 63 alkali and, 21-22 analysis. See Polysaccharide analysis chemical classification, 157, 179-180 0L-D-glucans, 165-168 [3-D-glucans, 168-173 fructans, 173 glycuronans, 173-179 chemical structure, 3-12 chemical substituents, 24-25, 28 concentration regimes, 71-74 mathematical modeling, 74-84 dispersions. See Dispersions enzymes and, 23-24 extraction, 123-125 films, 63, 73 flavor-releasing, 2-3 functional groups, 127-128 functions in food, 1-2 gamma-radiation and, 25 gelation, 10-11 gels. See Gels homogenization, 27 identification, 148 ionic, 43-47, 157 mathematical modeling concentration regimes, 74-84 molecular size, 85-100 micromolecules and, 25-27 molecular size, 85, 100 mathematical modeling, 85-100 molecular structure, 144-146 nonionic, 42-43, 157 oxidants and, 22-23 pastes, 62 phenomenology, 19-20 properties, 3-4, 27-28, 155 anisotropy, 17 cloudiness, 16, 55-56 coacervation, 67 colloidal activity, 12-18 configuration and conformation, 6-12, 28, 53 creep, 80, 82 degree of polymerization, 5-6 dispersibility, 12-13, 19, 38 dissociation, 44-45 Donnan distribution, 47, 135 elasticity, 16

233

Index

electric double layer, 45-46 electroosmosis, 47 electrophoresis, 46-47 electrostatics and electrokinetics, 42-47 electroviscosity, 57, 59 encapsulation, 68-69 flocculation, 65 foods, 64, 158-165 free volume, 54 hydration, 18, 29, 37, 69, 166 hydrodynamics, 53-54, 92-94, 191 hydrophilicity, 13-15, 146-147 hysteresis, 19, 82 ionization, 44-45 kinetics, 51-52 light scattering, 16-17 mass-volume-presure-temperature relationships, 41-42 molecular weight, 4-5, 85-100, 130-137 phase separation, 26, 67, 69 polymorphism, 18-19 pseudoplasticity, 74 rheology, 15-16, 56-59 sedimenation, 68, 95-98 stability, 21-22, 43, 64-69 state- and path-dependent properties, 41-69 strain hardening, 58 streaming potential, 47 surface area, 15, 98-99, 147 surfactancy, 17, 36 syneresis, 19, 67, 150 temperature dependence, 54-56 thermochemical stability, 118 thermodynamics, 47-51, 141-144 theta condition, 27, 151-154 thixotropy, 56, 74 turbidity, 16-17 variable-path processes, 59-64 viscoelasticity, 15, 16, 58, 74 viscosity, 15-16, 19, 57-58, 83, 103, 127, 189 volume, 27 volume fraction, 146 zeta potential, 45, 126-127 property and function modifications, 20-27 purification, 123-125 reactions [3-elimination, 21-22 controlled oxidation, 22 deesterification, 24, 28 enzyme hydrolysis, 23 sensory responses to, 3 sols, 55, 66

suspensions, 64 thermal analysis, 141-144 thermal processing, 115, 121 Amadori rearrangement, 120 atmospheric and retort processing, 115-118 caramels, 120-121 high-temperature pyrolysis, 119-120 low-temperature pyrolysis, 118-119 Maillard degradation, 120 strecker degradation, 120 uses, 1, 27-28, 148-165 xerogels. See Xerogels Polysaccharide-water interactions, 35-38, 116-118 Polysaccharide-water interface, 29-31, 35-40 Positional isomerism, 18 Positive adsorption, 38 Potato starch properties, 165, 166 water in, 35 Potential chemical, 32 interaction, 42 streaming, 46, 47 Zeta, 45 Power-law equation, 79 Propyleneglycol alginate, 24 properties, 161-163, 175, 180 uses in foods, 161-163 Protective colloid action, polysaccharides, 17, 36, 67 Protein-polysaccharide blends, 108, 109-112 Proteins, nature and properties, 108-109 Protopectin,.21, 125 Pseudocrystals, 9 Pseudoplasticity, 74 Pseudosolubility limit, 13, 25, 75 Pullalan, 168 Purification, polysaccharides, 123-125 Pyranoses, stability in acid, 21 Pyrolysis as analytical technique, 145 defined, 118 high-temperature, 119-120 low-temperature, 118-119

Q Quality control, pilot plants, 148-150, 155 Quaternary structure, 6

234

Index

R Radius hydrodynamic, 54, 89 of gyration, 5 Raffinose, solubility, 37 Random coil, 28, 55 conformation, 10, 69, 88, 101-102 Random conformations, 2 Random walk theory, 5 Raoult's law, 32, 96 Rayleigh ratio, 86 Rayleigh scattering, 86 Rayleigh' s law, 16-17 Reciprocal density, 92 Reducing end-group analysis, 131-132 Refractive index increment, 87 Regenerated cellulose (RC), 170 Reptation, 73 Retort processing, 115-118 Retrogradation, 11,166 Retrograded starch, 11 Reverse-phase HPLC, 129 Reversible gelation, 73, 83 Reynolds number, defined, 16 Rheology, polysaccharides, 15-16, 56-59 Rheometry, 83, 132-134, 155 Rheopexy, 56-57 Rice cake, 33 Rice fat mimetic, 187, 188 Rigidity modulus, 80 Root mean square mean-to-end distance, 90 Rotational viscometry, 79 Ruthenium red, 139

S Saccharides, 181 as fat replacement, 185-188 hemicellulose, 124, 181-182, 183 isolation, 183 oligosaccharides, 182-183 reactivity, 184, 185 uses, 184-185 Sacrificing agent, 63 Salt, polysaccharide dispersion and, 25-26 Salting in/salting out, 26, 46, 65 Scattering Debye, 86 dynamic light, 89 laser light, 137 monochromatic light, 37 Rayleigh, 86 wave vector, 89

Schulz-Blaschke equation, 78-79, 189 Schulz-Hardy rule, 56 Scleroglucan, gelation, 37 Second virial coefficient, 85 Sedimentation constant, 97 equilibrium, 137 polysaccharides, 68, 95-98 velocity, 137 Sedimentation equilibrium, 95-97, 137 Sedimentation velocity, 97-98, 137 Sediment volume, pilot plant quality control, 149-150 Segment factor, 45 Semidilute regime, 73-74 Shear-thickening, 56 Shear-thinning, 56 Shear viscosity, 15, 16 Shelf-life, 67, 69 Simple coacervation, 67 Simplesse, 186 Size exclusion chromatography, 134-135 Slippage, 75 Sodium, complex with polysaccharides, 107 Sol-gel transition, 55 Solka-floc, 169 Sols defned, 13 dilute, 72-73 phase changes, 55 stability, 66 Solubility, polysaccharides, 12 Soluble fiber, 147-148 Solution, 2, 50 Solvents, conformation and, 8-9 Specific heat, water, 34 Spectrophotometry, 138-140 Sphericity, 34, 35 Spray-drying, encapsulation method, 68 Stability chemical bond stability, 118 dispersion stability, 113, 118 polysaccharides aging and phase separation, 66-67, 69 electrostatic, 54 induced, 64 inherent, 64 kinetic, 64 steric, 65 thermodynamic, 64 Stabilization cryostabilization, 110 electrokinetic mechanism, 43 Stabilizers, 109

235

Index

Starch, 1 amorphous, 20 analysis, 138, 140 anisotropy, 17 chemical classification, 165-167 complementarity, 104 conformation, 9 crystalline forms, 19 depolymerization, 24 dispersion, 26 enzyme hydrolysis, 23 extraction and purification, 124 fermentation, 23 gelatinization, 52, 166 Hawaiian starchy food, 33 modified starches, 167-168 phenomenology, 20 retrograded, 11,167 stability in acid, 21 syneresis, 67 water activity, 33 water in, 35 Starch-konjac blends, synergism, 104 Starch-lipid complexes, 106 Stereoregularity, 7 Steric stabilization, 65 Stern layer, 45 Stokes equation, 75 Strain hardening, 58 Streaming potential, 47 Streamline flow, 15, 16 Strecker degradation, 120 Stress relaxation modulus, 80 Structure, 6 primary, 6 quaternary, 6 secondary, 6 tertiary, 6 Structural isomerism, 18 Substituted celluloses, 188 Sugar, polysaccharide dispersibility, 26 Sulfated glycans, chemical classification, 177-179 Supercooling, 41 Supercritical fluid chromatography (SFC), 145 Surface area, 15, 98-99, 147 Surface tension polysaccharides, 15 water, 34-35 Surfactancy, polysaccharides, 17, 36 Surfactant, 15, 34 Suspensions, polysaccharides, 64 Svedberg, 97 Sweeteners, 24 Swelling ratio, defined, 14

Syneresis defined, 67 pilot plant quality control, 150 polysaccharides, 19, 67 Syndiotactic distribution, 4 Synergism molecular basis, 105 polysaccharide-polysaccharide interaction, 104-105 polysaccharide-protein combinations, 110

T Temperature consolute, 55 critical, 55 gelation, 55 glass transition, 55, 166 melting, 55, 59 Temperature dependence, polysaccharides, 54 Texture, pilot plant quality control, 150 Thermal analysis, polysaccharides, 141-144 Thermal gravimetry (TG), 141, 142 Thermal processing, polysaccharides, 115, 121 Amadori rearrangement, 120 atmospheric and retort processing, 115-118 caramels, 120-121 high-temperature pyrolysis, 119-120 low-temperature pyrolysis, 118-119 Maillard degradation, 120 Strecker degradation, 120 Thermodynamics first law, 48 polysaccharides, 47-48 enthalpy, 48-49 entropy, 49 free energy of mixing, 49-51 irreversible, 51 Theta conditions, polysaccharides, 27, 151-154 Thin-layer chromatography (TLC), 129 Thixotropy, 56, 74 Time correlation function, 89 relaxation, 81, 82, 89 retardation, 82 Tinctorial value, 121 Total dietary fiber (TDF) assay, 148 Total energy content, 43 Tragacanthin, 175, 176 Traube's rule, 36 Triboelectrification, 63 Trouton rule, 16, 58

236

Index

Turbidimetry, 86 Turbidity, polysaccharides, 16 Tyndall effect, 16

U Universal graph, 92 Unperturbed chain reference, 93 Unperturbed dimension, 27 Upper cosolute temperature, 55

Vacuum infusion, of sugar into fruits and vegetables, 26-37 Van't Hoff equation, 85 Variable-path processes, polysaccharides, 59-64 Vegetables, 3 mealiness, 3 starch in, 165 vacuum infusion of sugar, 26-27 Viscoelasticity, 15, 58, 74, 75, 83, 84 polysaccharides, 15, 16, 58, 74 Viscometers, 77 Viscometry, 132-134 Viscosity complex viscosity, 83 elongational viscosity, 16 extensional viscosity, 16 polysaccharides, 15, 57-58 shear viscosity, 15, 16 units, 189 Viscosity blending chart, 103 Viscosity hysteresis, 19 Voigt-Kelvin model, 74, 80, 81, 191 Volume elution, 92 excluded, 31 fraction, 146 flee, 54 hydrodynamic, 92, 93 outer, 11, 53 partial molal, 33 partial specific, 96 polysaccharides, 27 specific, 98 void, 92 Volume fraction, 146

Water activity, 32, 33 coefficient of volume expansion, 34 chemical potential, 32 contraction and expansion, 34 dielectric constant, 31 free-draining, 53 hydrocolloidal, 35 ionization, 31-32 non-free-draining, 53, 68 of hydration, 14 plasticizing effect of, 14 properties, 31-35 specific heat, 34 surface tension, 34-35 Water activity, 32-34 Water of hydration, 14 Wetting, 36 White dextrin, 119, 182

X Xanthan-guar gum interaction, 113 Xanthan gum chemical classification, 176 colorimetry, 139 conformation, 11 hydration, 37 interaction with konjac mannan gums, 12 properties, 160-161,180 pseudoplasticity, 74 surfactancy, 17, 36 syneresis, 67 synergism, 104 uses in foods, 160-161, 164 Xanthan-locust bean gum, gelation, 105 Xanthation, 169 Xerogels, 62-63 defined, 62 hydration of, 19, 60 hydrophilicity, 13-15 rehydration, 60

Y Yeast, in fermentation, 23 Yellow dextrin, 119, 182

Z W Wall effects, 75

Zeta potential, 45, 126-127 Zimm plots, 88, 90

FOOD SCIENCE AND TECHNOLOGY

International Series Maynard A. Amerine, Rose Marie Pangborn, and Edward B. Roessler, Principles of Sensory Evaluation of Food. 1965. Martin Glicksman, Gum Technology in the Food Industry. 1970. Maynard A. Joslyn, Methods in Food Analysis, second edition. 1970. C. R. Stumbo, Thermobacteriology in Food Processing, second edition. 1973. Aaron M. Altschul (ed.), New Protein Foods: Volume 1, Technology, Part Am1974. Volume 2, Technology, Part B--1976. Volume 3, Animal Protein Supplies, Part A m 1978. Volume 4, Animal Protein Supplies, Part B m 1981. Volume 5, Seed Storage Proteins m 1985. S. A. Goldblith, L. Rey, and W. W. Rothmayr, FreezeDrying and Advanced Food Technology. 1975. R. B. Duckworth (ed.), Water Relations of Food. 1975. John A. Troller and J. H. B. Christian, Water Activity and Food. 1978. A. E. Bender, Food Processing and Nutrition. 1978. D. R. Osborne and P. Voogt, The Analysis of Nutrients in Foods. 1978. Marcel Loncin and R. L. Merson, Food Engineering: Principles and Selected Applications. 1979. J. G. Vaughan (ed.), Food Microscopy. 1979. J. R. A. Pollock (ed.), Brewing Science, Volume 1--1979. Volume 2m1980. Volume 3 - - 1987. J. Christopher Bauernfeind (ed.), Carotenoids as Colorants and Vitamin A Precursors: Technological and Nutritional Applications. 1981. Pericles Markakis (ed.), Anthocyanins as Food Colors. 1982. George F. Stewart and Maynard A. Amerine (eds.), Introduction to Food Science and Technology, second edition. 1982. Malcolm C. Bourne, Food Texture and Viscosity: Concept and Measurement. 1982. Hector A. Iglesias and Jorge Chirife, Handbook of Food Isotherms: Water Sorption Parameters for Food and Food Components. 1982. Colin Dennis (ed.), Post-Harvest Pathology of Fruits and Vegetables. 1983. P.J. Barnes (ed.), Lipids in Cereal Technology. 1983. David Pimentel and Carl W. Hall (eds.), Food and Energy Resources. 1984. Joe M. Regenstein and Carrie E. Regenstein, Food Protein Chemistry: An Introduction for Food Scientists. 1984. Maximo C. Gacula, Jr., and Jagbir Singh, Statistical Methods in Food and Consumer Research. 1984.

Fergus M. Clydesdale and Kathryn L. Wiemer (eds.), Iron Fortification of Foods. 1985. Robert V. Decareau, Microwaves in the Food Processing Industry. 1985. S. M. Herschdoerfer (ed.), Quality Control in the Food Industry, second edition. Volume 1-- 1985. Volume 2 - - 1985. Volume 3 - - 1986. Volume 4 - - 1987. F. E. Cunningham and N. A. Cox (eds.), Microbiology of Poultry Meat Products. 1987. Walter M. Urbain, Food Irradiation. 1986. Peter J. Bechtel, Muscle as Food. 1986. H. W.-S. Chan, Autoxidation of Unsaturated Lipids. 1986. Chester O. McCorkle, Jr., Economics of Food Processing in the United States. 1987. Jethro Jagtiani, Harvey T. Chan, Jr., and William S. Sakai, Tropical Fruit Processing. 1987. J. Solms, D. A. Booth, R. M. Dangborn, and O. Raunhardt, Food Acceptance and Nutrition. 1987. R. Macrae, HPLC in Food Analysis, second edition. 1988. A. M. Pearson and R. B. Young, Muscle and Meat Biochemistry. 1989. Dean O. Cliver (ed.), Foodborne Diseases. 1990. Marjorie P. Penfield and Ada Marie Campbell, Experimental Food Science, third edition. 1990. Leroy C. Blankenship, Colonization Control of Human Bacterial Enteropathogens in Poultry. 1991. Yeshajahu Pomeranz, Functional Properties of Food Components, second edition. 1991. Reginald H. Walter, The Chemistry and Technology of Pectin. 1991. Herbert Stone and Joel L. Sidel, Sensory Evaluation Practices, second edition. 1993. Robert L. Shewfelt and Stanley E. Prussia, Postharvest Handling: A Systems Approach. 1993. R. Paul Singh and Dennis R. Heldman, Introduction to Food Engineering, second edition. 1993. Tilak Nagodawithana and Gerald Reed, Enzymes in Food Processing, third edition. 1993. Dallas G. Hoover and Larry R. Steenson, Bacteriocins. 1993. Takayaki Shibamoto and Leonard Bjeldanes, Introduction to Food Toxicology. 1993. John A. Troller, Sanitation in Food Processing, second edition. 1993. Ronald S. Jackson, Wine Science: Principles and Applications. 1994. Harold D. Hafs and Robert G. Zimbelman, Low-fat Meats. 1994. Lance G. Phillips, Dana M. Whitehead, and John Kinsella, Structure-Function Properties of Food Proteins, 1994. Robert G. Jensen, Handbook of Milk Composition. 1995. Yrj5 H. Roos, Phase Transitions in Foods. 1995.